diff --git a/python/pydevSrc/com/jetbrains/python/debugger/PyDebugValue.java b/python/pydevSrc/com/jetbrains/python/debugger/PyDebugValue.java
index 5e5a37163ddb..32387d7d609c 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/PyDebugValue.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/PyDebugValue.java
@@ -9,7 +9,6 @@ import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.xdebugger.frame.*;
 import com.intellij.xdebugger.frame.presentation.XRegularValuePresentation;
 import com.jetbrains.python.debugger.pydev.PyDebugCallback;
-import com.jetbrains.python.debugger.pydev.PyVariableLocator;
 import com.jetbrains.python.debugger.render.PyNodeRenderer;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
@@ -23,7 +22,6 @@ import java.util.regex.Pattern;
 
 import static com.jetbrains.python.debugger.PyDebugValueGroupsKt.*;
 
-// todo: null modifier for modify modules, class objects etc.
 public class PyDebugValue extends XNamedValue {
   private static final Logger LOG = Logger.getInstance(PyDebugValue.class);
   private static final String DATA_FRAME = "DataFrame";
@@ -47,7 +45,6 @@ public class PyDebugValue extends XNamedValue {
   private @Nullable String myId = null;
   private ValuesPolicy myLoadValuePolicy;
   private @NotNull PyFrameAccessor myFrameAccessor;
-  private @Nullable PyVariableLocator myVariableLocator;
   private volatile @Nullable XValueNode myLastNode = null;
   private final boolean myErrorOnEval;
   private int myOffset;
@@ -117,7 +114,7 @@ public class PyDebugValue extends XNamedValue {
     myParent = parent;
     myFrameAccessor = frameAccessor;
     myLoadValuePolicy = ValuesPolicy.SYNC;
-    if (POLICY_DEFAULT_VALUES.keySet().contains(myValue)) {
+    if (POLICY_DEFAULT_VALUES.containsKey(myValue)) {
       myLoadValuePolicy = POLICY_DEFAULT_VALUES.get(myValue);
       setValue(" ");
     }
@@ -535,15 +532,6 @@ public class PyDebugValue extends XNamedValue {
     myFrameAccessor = frameAccessor;
   }
 
-  @Nullable
-  public PyVariableLocator getVariableLocator() {
-    return myVariableLocator;
-  }
-
-  public void setVariableLocator(@Nullable PyVariableLocator variableLocator) {
-    myVariableLocator = variableLocator;
-  }
-
   @Nullable
   public String getId() {
     return myId;
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/PyReferringObjectsValue.java b/python/pydevSrc/com/jetbrains/python/debugger/PyReferringObjectsValue.java
index 6527f29ed3d7..0f5b9270cd9d 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/PyReferringObjectsValue.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/PyReferringObjectsValue.java
@@ -20,11 +20,12 @@ import com.intellij.xdebugger.frame.XCompositeNode;
 import com.intellij.xdebugger.frame.XValueChildrenList;
 import com.jetbrains.python.debugger.pydev.PyDebugCallback;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 public class PyReferringObjectsValue extends PyDebugValue {
   private static final Logger LOG = Logger.getInstance(PyReferringObjectsValue.class);
 
-  private final @NotNull PyReferrersLoader myReferrersLoader;
+  private final @Nullable PyReferrersLoader myReferrersLoader;
 
   public PyReferringObjectsValue(@NotNull String name,
                                  String type,
@@ -52,7 +53,10 @@ public class PyReferringObjectsValue extends PyDebugValue {
   @Override
   public void computeChildren(@NotNull final XCompositeNode node) {
     if (node.isObsolete()) return;
-
+    if (myReferrersLoader == null) {
+      LOG.error("Failed to load Referring Objects. Frame accessor: " + getFrameAccessor());
+      return;
+    }
     myReferrersLoader.loadReferrers(this, new PyDebugCallback<XValueChildrenList>() {
       @Override
       public void ok(XValueChildrenList value) {
@@ -72,6 +76,6 @@ public class PyReferringObjectsValue extends PyDebugValue {
   }
 
   public boolean isField() {
-    return false; //TODO
+    return false;
   }
 }
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/PySourcePosition.java b/python/pydevSrc/com/jetbrains/python/debugger/PySourcePosition.java
index 8bb2760efb59..a8a0b4d30beb 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/PySourcePosition.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/PySourcePosition.java
@@ -17,7 +17,7 @@ public abstract class PySourcePosition {
   @Nullable
   protected String normalize(@Nullable String file) {
     if (file == null) {
-      return file;
+      return null;
     }
 
     return FileUtil.toSystemIndependentName(file);
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/PydevXmlUtils.java b/python/pydevSrc/com/jetbrains/python/debugger/PydevXmlUtils.java
index 9a2259ab2157..e40e41142863 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/PydevXmlUtils.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/PydevXmlUtils.java
@@ -23,7 +23,7 @@ import java.util.List;
  */
 public class PydevXmlUtils {
 
-  static SAXParserFactory parserFactory = SAXParserFactory.newInstance();
+  private static final SAXParserFactory parserFactory = SAXParserFactory.newInstance();
 
   private PydevXmlUtils() {
   }
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/ClientModeMultiProcessDebugger.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/ClientModeMultiProcessDebugger.java
index ce5b86204431..caf5301b6ff7 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/ClientModeMultiProcessDebugger.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/ClientModeMultiProcessDebugger.java
@@ -298,6 +298,7 @@ public class ClientModeMultiProcessDebugger implements ProcessDebugger {
     if (!isDebuggersEmpty()) {
       //here we add process id to thread name in case there are more then one process
       return Collections.unmodifiableCollection(Collections2.transform(threads, t -> {
+        if (t == null) return null;
         String threadName = ThreadRegistry.threadName(t.getName(), t.getId());
         PyThreadInfo newThread =
           new PyThreadInfo(t.getId(), threadName, t.getFrames(),
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetArrayCommand.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetArrayCommand.java
index 38ae0da92569..9f83fa9cef01 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetArrayCommand.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetArrayCommand.java
@@ -9,8 +9,6 @@ import org.jetbrains.annotations.NotNull;
  * @author amarch
  */
 public class GetArrayCommand extends GetFrameCommand {
-
-  private final PyDebugValue myParent;
   private final String myVariableName;
   private final int myRowOffset;
   private final int myColOffset;
@@ -27,7 +25,6 @@ public class GetArrayCommand extends GetFrameCommand {
     myRows = rows;
     myColumns = cols;
     myFormat = format;
-    myParent = var;
   }
 
   @Override
@@ -38,10 +35,7 @@ public class GetArrayCommand extends GetFrameCommand {
     payload.add(myColumns);
     payload.add(myFormat);
 
-    if (myParent.getVariableLocator() != null) {
-      payload.add(myParent.getVariableLocator().getThreadId()).add(myParent.getVariableLocator().getPyDBLocation());
-    }
-    else if (myVariableName.contains(GetVariableCommand.BY_ID)) {
+    if (myVariableName.contains(GetVariableCommand.BY_ID)) {
       //id instead of frame_id
       payload.add(getThreadId()).add(myVariableName);
     }
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetVariableCommand.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetVariableCommand.java
index 26578ed0a410..2e90f45e5ef8 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetVariableCommand.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/GetVariableCommand.java
@@ -48,10 +48,7 @@ public class GetVariableCommand extends GetFrameCommand {
 
   @Override
   protected void buildPayload(Payload payload) {
-    if (myParent.getVariableLocator() != null) {
-      payload.add(myParent.getVariableLocator().getThreadId()).add(myParent.getVariableLocator().getPyDBLocation());
-    }
-    else if (myVariableName.contains(BY_ID)) {
+    if (myVariableName.contains(BY_ID)) {
       //id instead of frame_id
       payload.add(getThreadId()).add(myVariableName);
     }
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/LoadFullValueCommand.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/LoadFullValueCommand.java
index dfbcf8477207..04270d1a3ca4 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/LoadFullValueCommand.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/LoadFullValueCommand.java
@@ -49,10 +49,7 @@ public class LoadFullValueCommand extends AbstractFrameCommand {
   private String buildPayloadForVar(@NotNull PyDebugValue var) {
     StringBuilder sb = new StringBuilder();
     String varName = GetVariableCommand.composeName(var);
-    if (var.getVariableLocator() != null) {
-      sb.append(var.getVariableLocator().getThreadId()).append(var.getVariableLocator().getPyDBLocation());
-    }
-    else if (varName.contains(GetVariableCommand.BY_ID)) {
+    if (varName.contains(GetVariableCommand.BY_ID)) {
       sb.append(getThreadId()).append(varName);
     }
     else {
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/MultiProcessDebugger.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/MultiProcessDebugger.java
index 8a579c7ada4c..017dd58602f1 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/MultiProcessDebugger.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/MultiProcessDebugger.java
@@ -53,6 +53,7 @@ public class MultiProcessDebugger implements ProcessDebugger {
       myDebugServerSocket = createServerSocket();
     }
     catch (ExecutionException e) {
+      LOG.error("Failed to start debugger:", e);
     }
     myMainDebugger = new RemoteDebugger(myDebugProcess, myDebugServerSocket, myTimeoutInMillis);
   }
@@ -85,18 +86,25 @@ public class MultiProcessDebugger implements ProcessDebugger {
     ApplicationManager.getApplication().executeOnPooledThread(myDebugProcessAcceptor);
   }
 
-  private static void sendDebuggerPort(Socket socket, ServerSocket serverSocket, IPyDebugProcess processHandler) throws IOException {
+  private static void sendDebuggerPort(@NotNull Socket socket, @NotNull ServerSocket serverSocket, @NotNull IPyDebugProcess processHandler)
+    throws IOException {
     int port = processHandler.handleDebugPort(serverSocket.getLocalPort());
     PrintWriter writer = new PrintWriter(socket.getOutputStream());
-    writer.println(99 + "\t" + -1 + "\t" + port);
-    writer.flush();
-    socket.close();
+    try {
+      writer.println(99 + "\t" + -1 + "\t" + port);
+      writer.flush();
+    }
+    finally {
+      socket.close();
+      writer.close();
+    }
   }
 
+  @NotNull
   private static ServerSocket createServerSocket() throws ExecutionException {
     final ServerSocket serverSocket;
     try {
-      //noinspection SocketOpenedButNotSafelyClosed
+      //noinspection IOResourceOpenedButNotSafelyClosed,SocketOpenedButNotSafelyClosed
       serverSocket = new ServerSocket(0);
     }
     catch (IOException e) {
@@ -263,6 +271,7 @@ public class MultiProcessDebugger implements ProcessDebugger {
     if (myOtherDebuggers.size() > 0) {
       //here we add process id to thread name in case there are more then one process
       return Collections.unmodifiableCollection(Collections2.transform(threads, t -> {
+        if (t == null) return null;
         String threadName = ThreadRegistry.threadName(t.getName(), t.getId());
         PyThreadInfo newThread =
           new PyThreadInfo(t.getId(), threadName, t.getFrames(),
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/RecurrentTaskExecutor.kt b/python/pydevSrc/com/jetbrains/python/debugger/pydev/RecurrentTaskExecutor.kt
index 058667008ebe..afd1d03431b5 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/RecurrentTaskExecutor.kt
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/RecurrentTaskExecutor.kt
@@ -77,7 +77,7 @@ class RecurrentTaskExecutor<out T>(threadsName: String,
   init {
     val threadFactory = ConcurrencyUtil.newNamedThreadFactory(threadsName)
     executorService = Executors.newFixedThreadPool(FIXED_THREAD_POOL_SIZE, threadFactory)
-    repeat(FIXED_THREAD_POOL_SIZE, { executorService.submit(TaskRunnable()) })
+    repeat(FIXED_THREAD_POOL_SIZE) { executorService.submit(TaskRunnable()) }
   }
 
   fun incrementRequests() {
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/RemoteDebugger.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/RemoteDebugger.java
index 3117abb48283..2c98f185e258 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/RemoteDebugger.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/RemoteDebugger.java
@@ -264,6 +264,10 @@ public class RemoteDebugger implements ProcessDebugger {
   // todo: change variable in lists doesn't work - either fix in pydevd or format var name appropriately
   private void setTempVariable(final String threadId, final String frameId, final PyDebugValue var) {
     final PyDebugValue topVar = var.getTopParent();
+    if (topVar == null) {
+      LOG.error("Top parent is null");
+      return;
+    }
     if (!myDebugProcess.canSaveToTemp(topVar.getName())) {
       return;
     }
diff --git a/python/pydevSrc/com/jetbrains/python/debugger/pydev/transport/ServerModeDebuggerTransport.java b/python/pydevSrc/com/jetbrains/python/debugger/pydev/transport/ServerModeDebuggerTransport.java
index f5c69c0c9825..16d45b19646d 100644
--- a/python/pydevSrc/com/jetbrains/python/debugger/pydev/transport/ServerModeDebuggerTransport.java
+++ b/python/pydevSrc/com/jetbrains/python/debugger/pydev/transport/ServerModeDebuggerTransport.java
@@ -125,7 +125,7 @@ public class ServerModeDebuggerTransport extends BaseDebuggerTransport {
   }
 
   public static class DebuggerReader extends BaseDebuggerReader {
-    public DebuggerReader(@NotNull RemoteDebugger debugger, @NotNull InputStream stream) throws IOException {
+    public DebuggerReader(@NotNull RemoteDebugger debugger, @NotNull InputStream stream) {
       super(stream, StandardCharsets.UTF_8, debugger); //TODO: correct encoding?
       start(getClass().getName());
     }
diff --git a/python/src/com/jetbrains/python/debugger/PyDebugProcess.java b/python/src/com/jetbrains/python/debugger/PyDebugProcess.java
index 6edc9fc36cd8..07822797b6dc 100644
--- a/python/src/com/jetbrains/python/debugger/PyDebugProcess.java
+++ b/python/src/com/jetbrains/python/debugger/PyDebugProcess.java
@@ -75,8 +75,6 @@ import java.util.concurrent.ConcurrentHashMap;
 /**
  * @author yole
  */
-// todo: bundle messages
-// todo: pydevd supports module reloading - look for a way to use the feature
 public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, ProcessListener {
 
   private static final Logger LOG = Logger.getInstance(PyDebugProcess.class);
@@ -100,8 +98,6 @@ public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, Pr
   private final Map<String, PyDebugValue> myNewVariableValue = Maps.newHashMap();
   private boolean myDownloadSources = false;
 
-  private boolean myClosing = false;
-
   protected PyPositionConverter myPositionConverter;
   private final XSmartStepIntoHandler<?> mySmartStepIntoHandler;
   private boolean myWaitingForConnection = false;
@@ -424,16 +420,16 @@ public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, Pr
     if ("@@BUILD_NUMBER@@".equals(remoteVersion)) {
       remoteVersion = currentBuild;
     }
-    else remoteVersion = StringUtil.trimStart(remoteVersion, "PY-");
+    else {
+      remoteVersion = StringUtil.trimStart(remoteVersion, "PY-");
+    }
     printToConsole("Connected to pydev debugger (build " + remoteVersion + ")\n", ConsoleViewContentType.SYSTEM_OUTPUT);
 
-    if (remoteVersion != null) {
-      if (!(remoteVersion.equals(currentBuild) || remoteVersion.startsWith(currentBuild))) {
-        LOG.warn(String.format("Wrong debugger version. Remote version: %s Current build: %s", remoteVersion, currentBuild));
-        printToConsole(String.format("Warning: wrong debugger version. Use pycharm-debugger.egg from PyCharm installation folder\n" +
-                       "Or execute: 'pip install pydevd-pycharm~=%s'\n", currentBuild),
-                       ConsoleViewContentType.ERROR_OUTPUT);
-      }
+    if (!(remoteVersion.equals(currentBuild) || remoteVersion.startsWith(currentBuild))) {
+      LOG.warn(String.format("Wrong debugger version. Remote version: %s Current build: %s", remoteVersion, currentBuild));
+      printToConsole(String.format("Warning: wrong debugger version. Use pycharm-debugger.egg from PyCharm installation folder\n" +
+                                   "Or execute: 'pip install pydevd-pycharm~=%s'\n", currentBuild),
+                     ConsoleViewContentType.ERROR_OUTPUT);
     }
   }
 
@@ -564,7 +560,7 @@ public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, Pr
 
   @Override
   public void resume(@Nullable XSuspendContext context) {
-    passToAllThreads(ResumeOrStepCommand.Mode.RESUME);
+    passResumeToAllThreads();
   }
 
   @Override
@@ -611,11 +607,11 @@ public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, Pr
     return false;
   }
 
-  private void passToAllThreads(final ResumeOrStepCommand.Mode mode) {
+  private void passResumeToAllThreads() {
     dropFrameCaches();
     if (isConnected()) {
       for (PyThreadInfo thread : myDebugger.getThreads()) {
-        myDebugger.resumeOrStep(thread.getId(), mode);
+        myDebugger.resumeOrStep(thread.getId(), ResumeOrStepCommand.Mode.RESUME);
       }
     }
   }
@@ -1060,7 +1056,6 @@ public class PyDebugProcess extends XDebugProcess implements IPyDebugProcess, Pr
 
   @Override
   public void processWillTerminate(@NotNull ProcessEvent event, boolean willBeDestroyed) {
-    myClosing = true;
   }
 
   @Override