reset FocusWatcher.myTopComponent to null

The field is set in install() and should probably be reset symmetrically in deinstall(), otherwise it may hold UI objects.

platform/util/src/com/intellij/openapi/wm/FocusWatcher.java

@@ -89,6 +89,7 @@ public class FocusWatcher implements ContainerListener,FocusListener{
     if(getFocusedComponent() ==component){
       setFocusedComponentImpl(null, cause);
     }
+    myTopComponent = null;
   }
 
   @Override