Modified to initialize variables in _onDrop. so destory function does not work well.

Sortable.js

@@ -808,6 +808,11 @@
 					}
 				}
 
+			}
+			this._nulling();
+		},
+
+		_nulling: function() {
 			// Nulling
 			rootEl =
 			dragEl =
@@ -830,10 +835,8 @@
 
 			activeGroup =
 			Sortable.active = null;
-			}
 		},
 
-
 		handleEvent: function (/**Event*/evt) {
 			var type = evt.type;