Datepicker: Fix this reference in _hideDatepicker(). Fixed #7722 - Datepicker: Does not properly clear current instance when hidden.

(cherry picked from commit c36b43d)

Author: Jay Oster

ui/jquery.ui.datepicker.js

@@ -797,9 +797,10 @@ $.extend(Datepicker.prototype, {
 		if (this._datepickerShowing) {
 			var showAnim = this._get(inst, 'showAnim');
 			var duration = this._get(inst, 'duration');
+			var self = this;
 			var postProcess = function() {
 				$.datepicker._tidyDialog(inst);
-				this._curInst = null;
+				self._curInst = null;
 			};
 			if ($.effects && $.effects[showAnim])
 				inst.dpDiv.hide(showAnim, $.datepicker._get(inst, 'showOptions'), duration, postProcess);