Ext.form.ComboBox 空选择解决方案

var _testComboBox = new Ext.form.ComboBox({       hiddenName : "test",       displayField : "test",       fieldLabel : "测试",       width : 100,       mode : "local",       readOnly : true,       triggerAction : "all",       listeners : {        'select' : {         fn : function() {          if (_testComboBox.getValue() == "&nbsp;") {           _testComboBox.setValue("")          }         },         scope : this        }       },       store : new Ext.data.SimpleStore({          fields : ["test"],          data : [["&nbsp;"], ["1"], ["2"]]         })      });    var _form = new Ext.FormPanel({       items : _testComboBox      }) var test = new Ext.Window({       title : "<center><font size=2>测试</font></center>",       closeAction : 'hide',       layout : 'border',       width : 850,       height : 500,       draggable : false,       resizable : false,       items : [{          region : 'center',          frame : true,          items : _form }]      }).show()