Add [NullAllowed] to NSWindow.Toolbar (#2738)

- https://bugzilla.xamarin.com/show_bug.cgi?id=59635

src/appkit.cs

@@ -20090,6 +20090,7 @@ namespace XamCore.AppKit {
 		void RecalculateKeyViewLoop ();
 	
 		[Export ("toolbar")]
+		[NullAllowed]
 		NSToolbar Toolbar { get; set; }
 	
 		[Export ("toggleToolbarShown:")]