[macOS] Possible NPE in Menu._setVisible

[tmssngr]

Describe the bug
SmartGit users report following exception:

java.lang.NullPointerException
	at org.eclipse.swt.widgets.Menu._setVisible(SourceFile:271)
	at org.eclipse.swt.widgets.Display.runPopups(SourceFile:4490)
	at org.eclipse.swt.widgets.Display.readAndDispatch(SourceFile:3982)
...

It is following code:

		// Hold on to window in case it is disposed while the popup is open.
		window.retain();
		// NSMenu processes events on its own while the popup is open.
		display.sendPreExternalEventDispatchEvent ();
		NSEvent nsEvent = NSEvent.otherEventWithType(OS.NSApplicationDefined, location, 0, 0.0, window.windowNumber(), window.graphicsContext(), (short)0, 0, 0);
		NSMenu.popUpContextMenu(nsMenu, nsEvent, shell.view);
		display.sendPostExternalEventDispatchEvent (); // << here NPE
		window.release();

To Reproduce
Not reproducible reliably.

Expected behavior
No exception.

Screenshots

Environment:

1. Select the platform(s) on which the behavior is seen:

• • All OS
• • Windows
• • Linux
• • macOS

[tmssngr]

Can someone please review my PR?

[mickaelistria]

Closed with #923