mash: Fix crash when clicking on the shelf overflow icon

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index 517de7f6d9b21ea81d085aef6cedb6a49bd883d2..e37ec6c2aa9fb5bb190feb69e8dfe8fad7742d3a 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -77,6 +77,10 @@ class AshTouchExplorationManager;
 // 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.