CrOS Shelf: Prevent a crash when calculating hotseat state during shutdown

Change-Id: I4ab18dcab81d8622a52b9b744bd7aa57ae1693da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911220Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714499}

ash/shelf/shelf_layout_manager.cc

@@ -1079,6 +1079,10 @@ HotseatState ShelfLayoutManager::CalculateHotseatState(
     return HotseatState::kShown;
 
   auto* app_list_controller = Shell::Get()->app_list_controller();
+  // If the app list controller is null, we are probably in the middle of
+  // a shutdown, let's not change the hotseat state.
+  if (!app_list_controller)
+    return hotseat_state();
   const auto* overview_controller = Shell::Get()->overview_controller();
   const bool in_overview =
       ((overview_controller && overview_controller->InOverviewSession()) ||