ExtJs面向对象错误

this.ComboFunction=new Business.sys.ComboFunction({
                id:'FunctionId',
                key:'FunctionId' 
            });

GM.FormItem= function(config){
    this.config = config||{}; 
    this.ComboFunction=new Business.sys.ComboFunction({
                id:'FunctionId',
                key:'FunctionId' 
            });
    var defConfig = {
     frame: true, 
     labelWidth: 100,         
     defaults: { 
   layout: "form", 
   xtype: 'textfield'
  },
        items:[{
    key:'ID',       
    fieldLabel: '帮助ID',
    allowBlank:true,
    readOnly:true
   },{
    key:'SortId',       
    xtype:'hidden'
   },{         
    fieldLabel: '标题',
    allowBlank:false,
    maxLength:36,
    key: 'Title'            
   },{         
    fieldLabel: '所属类别',
    maxLength:36,
    readOnly:true,
    key: 'SortTitle'           
   },{         
    fieldLabel: '关键字',  
    maxLength:36,
    key: 'Keywords'           
   },this.ComboFunction,{
    hideLabel:true,
    fieldLabel: '内容',
    xtype:'bhtmleditor',
    height:500,
    key: 'Content'
  }]
    };
    Ext.applyIf(this.config, defConfig);
    GM.FormItem.superclass.constructor.call(this, this.config);alert(this.ComboFunction);
}

在创建窗体第一次显示，第二次以后就不显示combo