[Mac] Fix NSPopover Delegate disposing
[Mac] Fix NSPopover Delegate disposing

DidClose is not guaranteed to be executed before
the Popover or its Delegate have been Disposed
and/or GCd and may crash.

cra0zy/xwt,hamekoz/xwt,hwthomas/xwt,mono/xwt,antmicro/xwt,lytico/xwt,TheBrainTech/xwt