Themes: Avoid a JS error in the keyup event handler when the overlay is not yet initialized.

fixes #26497.

Built from https://develop.svn.wordpress.org/trunk@26826


git-svn-id: http://core.svn.wordpress.org/trunk@26713 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/theme.js

@@ -390,6 +390,10 @@ themes.view.Themes = wp.Backbone.View.extend({
 
 		// Bind keyboard events.
 		$('body').on( 'keyup', function( event ) {
+			if ( ! self.overlay ) {
+				return;
+			}
+
 			// Pressing the right arrow key fires a theme:next event
 			if ( event.keyCode === 39 ) {
 				self.overlay.nextTheme();