Fix null dereference in shelf views

Tablet mode controller is destroyed slightly before shelf widget in
Shell destructor. Nullptr check is needed, because IsTabletModeEnabled()
can get called when shelf is being destroyed.

Bug: 967149
Change-Id: I42b6abd6cf24902cc1647d03dd813b070d9aec05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637982Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Aga Wronska <agawronska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665126}

ash/shelf/default_shelf_view.cc

@@ -21,9 +21,12 @@ namespace ash {
 namespace {
 
 bool IsTabletModeEnabled() {
-  return Shell::Get()
-      ->tablet_mode_controller()
-      ->IsTabletModeWindowManagerEnabled();
+  // This check is needed, because tablet mode controller is destroyed before
+  // shelf widget. See https://crbug.com/967149 for more details.
+  return Shell::Get()->tablet_mode_controller() &&
+         Shell::Get()
+             ->tablet_mode_controller()
+             ->IsTabletModeWindowManagerEnabled();
 }
 
 }  // namespace

ash/shelf/kiosk_next_shelf_view.cc

@@ -90,9 +90,12 @@ class KioskNextHomeButton : public AppListButton {
 };
 
 bool IsTabletModeEnabled() {
-  return Shell::Get()
-      ->tablet_mode_controller()
-      ->IsTabletModeWindowManagerEnabled();
+  // This check is needed, because tablet mode controller is destroyed before
+  // shelf widget. See https://crbug.com/967149 for more details.
+  return Shell::Get()->tablet_mode_controller() &&
+         Shell::Get()
+             ->tablet_mode_controller()
+             ->IsTabletModeWindowManagerEnabled();
 }
 
 }  // namespace