Media: Account for a race condition when deleting an edit gallery frame. see #21390.

git-svn-id: http://core.svn.wordpress.org/trunk@22530 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/mce-view.js

@@ -740,6 +740,7 @@ window.wp = window.wp || {};
 				// Create a single-use frame. If the frame is closed,
 				// then detach it from the DOM and remove the reference.
 				this.frame.on( 'close', function() {
+					if ( this.frame )
 						this.frame.detach();
 					delete this.frame;
 				}, this );