Fix crash in proactive suggestions.

Hover changed events may be propagated during the widget close
sequence. Previously, this could cause us to schedule a timer to auto-
close the proactive suggestions view which would crash when firing due
to the widget no longer existing.

Bug: b:141386438
Change-Id: Ia2a2cf8ba11c9f1209df857edccbd160e1be45c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1817762Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#699078}

ash/assistant/assistant_ui_controller.cc

@@ -300,6 +300,15 @@ void AssistantUiController::OnProactiveSuggestionsCloseButtonPressed() {
 
 void AssistantUiController::OnProactiveSuggestionsViewHoverChanged(
     bool is_hovering) {
+  if (!proactive_suggestions_view_ ||
+      !proactive_suggestions_view_->GetWidget() ||
+      proactive_suggestions_view_->GetWidget()->IsClosed()) {
+    // Hover changed events may occur during the proactive suggestions widget's
+    // close sequence. When this occurs, we quit early as the proactive
+    // suggestions view is being destroyed.
+    return;
+  }
+
   if (!is_hovering) {
     // When the user is no longer hovering over the proactive suggestions view
     // we need to reset the timer so that it will auto-close appropriately.