fix for nested sortables with react-sortablejs

Could you add this conditional so that I can use nested sortables with react-sortablejs?
https://github.com/cheton/react-sortable/issues/23

Sortable.js

@@ -1338,7 +1338,9 @@
 
 		evt.originalEvent = originalEvt;
 
-		rootEl.dispatchEvent(evt);
+		if(rootEl) {
+		  rootEl.dispatchEvent(evt);
+	        }
 
 		if (options[onName]) {
 			options[onName].call(sortable, evt);