Fix Quartz Full Screen Crash.

gdk/quartz/GdkQuartzView.c

@@ -684,7 +684,7 @@
   GdkWindowImplQuartz *impl = GDK_WINDOW_IMPL_QUARTZ (private->impl);
   NSRect rect;
 
-  if (!impl->toplevel)
+  if (!impl || !impl->toplevel)
     return;
 
   if (trackingRect)

gdk/quartz/GdkQuartzWindow.c

@@ -24,6 +24,14 @@
 
 @implementation GdkQuartzWindow
 
+- (void)windowWillClose:(NSNotification*)notification
+{
+  // Clears the delegate when window is going to be closed; since EL
+  // Capitan it is possible that the methods of delegate would get
+  // called after the window has been closed.
+  [self setDelegate:nil];
+}
+
 -(BOOL)windowShouldClose:(id)sender
 {
   GdkWindow *window = [[self contentView] gdkWindow];