overview: Fix crash when closing windows.

Test: manual
Bug: 839290
Change-Id: Ib72e5b43a1a8a0e7dd429dbef90de9b6b1b820c6
Reviewed-on: https://chromium-review.googlesource.com/1042809Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555839}

ash/wm/overview/window_grid.cc

@@ -493,7 +493,7 @@ WindowSelectorItem* WindowGrid::SelectedWindow() const {
 WindowSelectorItem* WindowGrid::GetWindowSelectorItemContaining(
     const aura::Window* window) const {
   for (const auto& window_item : window_list_) {
-    if (window_item->Contains(window))
+    if (window_item && window_item->Contains(window))
       return window_item.get();
   }
   return nullptr;