fixing ChooseByNameBase assertions

* list update is requested
* bg thread is canceled, a new one is started
* then quickly a write action occurs
* old bg thread hasn't finished yet and receives that event and restarts
=> two bg threads at the same time, bad

platform/lang-impl/src/com/intellij/ide/util/gotoByName/ChooseByNameBase.java

@@ -1521,8 +1521,9 @@ public abstract class ChooseByNameBase {
                   ApplicationAdapter listener = new ApplicationAdapter() {
                     @Override
                     public void beforeWriteActionStart(Object action) {
-                      cancel();
-                      scheduleRestart();
+                      if (cancel()) {
+                        scheduleRestart(); //don't restart if already canceled explicitly
+                      }
                       ApplicationManager.getApplication().removeApplicationListener(this);
                     }
                   };