Return from OnWindowActivated if widget has been destroyed

Bug: 1021639
Change-Id: If65812f8a2d961f88fc66c9b060671e7b7d8e8ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900293Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713291}

ash/system/unified/unified_system_tray_bubble.cc

@@ -301,7 +301,7 @@ void UnifiedSystemTrayBubble::OnWidgetDestroying(views::Widget* widget) {
 void UnifiedSystemTrayBubble::OnWindowActivated(ActivationReason reason,
                                                 aura::Window* gained_active,
                                                 aura::Window* lost_active) {
-  if (!gained_active)
+  if (!gained_active || !bubble_widget_)
     return;
 
   // Don't close the bubble if a transient child is gaining or losing