Media manager: Avoid a blank modal when an invalid image size class is set on the image.

props gcorne.
fixes #24409.

Built from https://develop.svn.wordpress.org/trunk@27687


git-svn-id: http://core.svn.wordpress.org/trunk@27526 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/media-models.js

@@ -448,6 +448,11 @@ window.wp = window.wp || {};
 			}
 
 			size = this.attachment.get( 'sizes' )[ this.get( 'size' ) ];
+
+			if ( ! size ) {
+				return;
+			}
+
 			this.set( 'url', size.url );
 			this.set( 'width', size.width );
 			this.set( 'height', size.height );