Widget: Make contextless widget construction work

Due to the fact the widget factory code is now in strict mode, the check for
being called without using the `new` keyword started breaking if you save the
widget constructor to a variable before calling it:
```js
var customWidget = $.custom.customWidget;
customWidget( {}, elem );
```
as then `this` is undefined and checking for `this._createWidget` crashes.
Account for that with an additional check.

Fixes gh-2015
Closes gh-2019

1 files changed, 1 insertions, 1 deletions

diff --git a/ui/widget.js b/ui/widget.js
index 3b149f11a..04daaa883 100644
--- a/ui/widget.js
+++ b/ui/widget.js
@@ -77,7 +77,7 @@ $.widget = function( name, base, prototype ) {
 	constructor = $[ namespace ][ name ] = function( options, element ) {
 
 		// Allow instantiation without "new" keyword
-		if ( !this._createWidget ) {
+		if ( !this || !this._createWidget ) {
 			return new constructor( options, element );
 		}