Fixed PIEAE when imported file is renamed and then renamed back using undo

ExportedNameCache contains elements with parent file stubs without the PSI element
set. Therefore these elements are invalid. We reset exported name cache each time
the modification stamp differs from its value at the moment of cache creation,

python/src/com/jetbrains/python/psi/impl/PyFileImpl.java

@@ -54,8 +54,10 @@ public class PyFileImpl extends PsiFileBase implements PyFile, PyExpression {
     private final List<PsiElement> myNameDefiners = new ArrayList<PsiElement>();
     private final List<String> myNameDefinerNegativeCache = new ArrayList<String>();
     private long myNameDefinerOOCBModCount = -1;
+    private final long myModificationStamp;
 
-    private ExportedNameCache() {
+    private ExportedNameCache(long modificationStamp) {
+      myModificationStamp = modificationStamp;
       final List<PsiElement> children = PyPsiUtils.collectAllStubChildren(PyFileImpl.this, getStub());
       final List<PyExceptPart> exceptParts = new ArrayList<PyExceptPart>();
       for (PsiElement child : children) {
@@ -226,6 +228,9 @@ public class PyFileImpl extends PsiFileBase implements PyFile, PyExpression {
       return null;
     }
 
+    public long getModificationStamp() {
+      return myModificationStamp;
+    }
   }
 
   public PyFileImpl(FileViewProvider viewProvider) {
@@ -433,8 +438,13 @@ public class PyFileImpl extends PsiFileBase implements PyFile, PyExpression {
     ExportedNameCache cache;
     synchronized (myENCLock) {
       cache = myExportedNameCache != null ? myExportedNameCache.get() : null;
+      final long modificationStamp = getModificationStamp();
+      if (myExportedNameCache != null && cache != null && modificationStamp != cache.getModificationStamp()) {
+        myExportedNameCache.clear();
+        cache = null;
+      }
       if (cache == null) {
-        cache = new ExportedNameCache();
+        cache = new ExportedNameCache(modificationStamp);
         myExportedNameCache = new SoftReference<ExportedNameCache>(cache);
       }
     }