mash: Fix crash when clicking on the shelf overflow icon

ash/root_window_controller.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_ROOT_WINDOW_CONTROLLER_H_
 #define ASH_ROOT_WINDOW_CONTROLLER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 59 matching lines @@
 class AshTouchExplorationManager;
 #endif
 
 // This class maintains the per root window state for ash. This class
 // owns the root window and other dependent objects that should be
 // deleted upon the deletion of the root window. This object is
 // indirectly owned and deleted by |WindowTreeHostManager|.
 // The RootWindowController for particular root window is stored in
 // its property (RootWindowSettings) and can be obtained using
 // |GetRootWindowController(aura::WindowEventDispatcher*)| function.
+//
+// NOTE: In classic ash there is one RootWindow per display, so every RootWindow
+// has a RootWindowController. In mus/mash there is one RootWindow per top-level
+// Widget, so not all RootWindows have a RootWindowController.
 class ASH_EXPORT RootWindowController : public ShellObserver {
  public:
   // Creates and Initialize the RootWindowController for primary display.
   static void CreateForPrimaryDisplay(AshWindowTreeHost* host);
 
   // Creates and Initialize the RootWindowController for secondary displays.
   static void CreateForSecondaryDisplay(AshWindowTreeHost* host);
 
   // Returns a RootWindowController of the window's root window.
   static RootWindowController* ForWindow(const aura::Window* window);
@@ skipping 216 matching lines @@
 };
 
 
 // Gets the RootWindowController for |root_window|.
 ASH_EXPORT RootWindowController* GetRootWindowController(
     const aura::Window* root_window);
 
 }  // namespace ash
 
 #endif  // ASH_ROOT_WINDOW_CONTROLLER_H_