Pressing Alt key on a Toplevel with only a MenuBar throws System.InvalidOperationException.

[BDisp]

Like #2776, running this unit test will fail on developer branch:

		[Fact, AutoInitShutdown]
		public void Activating_MenuBar_By_Alt_Key_Does_Not_Throw ()
		{
			var menu = new MenuBar (new MenuBarItem [] {
				new MenuBarItem ("Child", new MenuItem [] {
					new MenuItem ("_Create Child", "", null)
				})
			});
			var topChild = new Toplevel ();
			topChild.Add (menu);
			Application.Top.Add (topChild);
			Application.Begin (Application.Top);

			var exception = Record.Exception (() => topChild.ProcessHotKey (new KeyEvent (Key.AltMask, new KeyModifiers { Alt = true })));
			Assert.Null (exception);
		}