Fixed NSException raised when using Cocoa windows after the SFML window has been closed.

git-svn-id: https://sfml.svn.sourceforge.net/svnroot/sfml/trunk@1482 4e206d99-4929-0410-ac5d-dfc041789085

src/SFML/Window/Cocoa/GLKit.mm

@@ -549,6 +549,7 @@
 		[myWindow release];
 		myWindow = [aWindow retain];
 		
+		if (myWindow) {
 			// Set the new observers
 			// We want to know when our window got the focus
 			[nc addObserver:self
@@ -567,7 +568,7 @@
 				   selector:@selector(windowWillClose:)
 					   name:NSWindowWillCloseNotification
 					 object:[self window]];
-		
+		}
 	}
 }