Add machinery to TrackList for pending changes to tracks

src/AudacityApp.cpp

@@ -1087,6 +1087,9 @@ bool AudacityApp::OnExceptionInMainLoop()
          // failed operation
          pProject->RollbackState();
 
+         // Forget pending changes in the TrackList
+         pProject->GetTracks()->ClearPendingTracks();
+
          pProject->RedrawProject();
 
          // Give the user an alert