Fix Chrome would crash when a tab is closed with migration dialog

being shown.

Bug: 897998
Change-Id: I884709065290889f494a02e06d28602f2bcee68c
Reviewed-on: https://chromium-review.googlesource.com/c/1325561Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607036}

chrome/browser/ui/views/autofill/local_card_migration_dialog_view.cc

@@ -306,6 +306,7 @@ void LocalCardMigrationDialogView::ShowDialog() {
 }
 
 void LocalCardMigrationDialogView::CloseDialog() {
+  controller_ = nullptr;
   GetWidget()->Close();
 }
 
@@ -370,7 +371,10 @@ bool LocalCardMigrationDialogView::Cancel() {
 }
 
 void LocalCardMigrationDialogView::WindowClosing() {
-  controller_->OnDialogClosed();
+  if (controller_) {
+    controller_->OnDialogClosed();
+    controller_ = nullptr;
+  }
 }
 
 // TODO(crbug/867194): Add button pressed logic for kDeleteCardButtonTag.