Remove itself from the widget observers in destructor

ui/message_center/views/message_center_bubble.cc

Index: ui/message_center/views/message_center_bubble.cc
diff --git a/ui/message_center/views/message_center_bubble.cc b/ui/message_center/views/message_center_bubble.cc
index 62935c0c0616266841c99549a610bcf759c0fe94..d922c78f7926705fa47148da1397c02c70cba959 100644
--- a/ui/message_center/views/message_center_bubble.cc
+++ b/ui/message_center/views/message_center_bubble.cc
@@ -68,6 +68,10 @@ MessageCenterBubble::MessageCenterBubble(MessageCenter* message_center,
       first_item_has_no_margin_(first_item_has_no_margin) {}
 
 MessageCenterBubble::~MessageCenterBubble() {
+  // Removs this from the widget observers just in case. MessageCenterBubble
+  // might be destoryed without calling its Widget's Close/CloseNow.
+  if (bubble_view() && bubble_view()->GetWidget())
+    bubble_view()->GetWidget()->RemoveObserver(this);
 }
 
 void MessageCenterBubble::SetSettingsVisible() {
@@ -120,7 +124,8 @@ void MessageCenterBubble::UpdateBubbleView() {
 }
 
 void MessageCenterBubble::OnWidgetClosing(views::Widget* widget) {
-  bubble_view()->GetWidget()->RemoveObserver(this);
+  if (bubble_view() && bubble_view()->GetWidget())
+    bubble_view()->GetWidget()->RemoveObserver(this);
   if (message_center_view_)
     message_center_view_->SetIsClosing(true);
 }