[Mac] Fix System.NullReferenceException in MenuBackend.Popup.
[Mac] Fix System.NullReferenceException in MenuBackend.Popup.

Xwt.Mac.MenuBackend.Popup relies on NSApplication.SharedApplication.CurrentEvent, which can be null.
Example: User activates menu using VoiceOver and Ctr+Option+Space combination.

hwthomas/xwt,TheBrainTech/xwt,antmicro/xwt,lytico/xwt,mono/xwt