diff --git a/platform/testFramework/src/com/intellij/testFramework/fixtures/impl/CodeInsightTestFixtureImpl.java b/platform/testFramework/src/com/intellij/testFramework/fixtures/impl/CodeInsightTestFixtureImpl.java
index d21b2136fcc0..e20ba1965d8a 100644
--- a/platform/testFramework/src/com/intellij/testFramework/fixtures/impl/CodeInsightTestFixtureImpl.java
+++ b/platform/testFramework/src/com/intellij/testFramework/fixtures/impl/CodeInsightTestFixtureImpl.java
@@ -127,6 +127,7 @@ import com.intellij.util.ui.UIUtil;
 import junit.framework.ComparisonFailure;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.TestOnly;
 
 import javax.swing.*;
 import java.io.File;
@@ -168,10 +169,7 @@ public class CodeInsightTestFixtureImpl extends BaseFixture implements CodeInsig
   private static void addGutterIconRenderer(GutterMark renderer, int offset, @NotNull SortedMap<Integer, List<GutterMark>> result) {
     if (renderer == null) return;
 
-    List<GutterMark> renderers = result.get(offset);
-    if (renderers == null) {
-      result.put(offset, renderers = new SmartList<>());
-    }
+    List<GutterMark> renderers = result.computeIfAbsent(offset, k -> new SmartList<>());
     renderers.add(renderer);
   }
 
@@ -195,10 +193,12 @@ public class CodeInsightTestFixtureImpl extends BaseFixture implements CodeInsig
   }
 
   @NotNull
+  @TestOnly
   public static List<HighlightInfo> instantiateAndRun(@NotNull PsiFile file,
                                                       @NotNull Editor editor,
                                                       @NotNull int[] toIgnore,
                                                       boolean canChangeDocument) {
+    ApplicationManager.getApplication().assertIsDispatchThread();
     Project project = file.getProject();
     ensureIndexesUpToDate(project);
     DaemonCodeAnalyzerImpl codeAnalyzer = (DaemonCodeAnalyzerImpl)DaemonCodeAnalyzer.getInstance(project);