将combobox与远程对象链接起来很麻烦

Question

我在数据网格中有一个组合框，从远程对象读取其值。
我想显示提示或设置默认值，因为如果我打开组合框并且我没有选择任何东西我会崩溃。
这是代码：

 <mx:DataGridColumn  dataField="teacher" width="170" editable="true" editorDataField="selectedItem"  >
                            <mx:itemEditor>
                                <fx:Component>
                                    <mx:ComboBox dataProvider="{this.outerDocument.staffnames}" >
                                    </mx:ComboBox>
                                </fx:Component>   
                            </mx:itemEditor>

                        </mx:DataGridColumn>

远程对象：

        [Bindable] public var staffnames:Array = new Array();


public function getAllStaff_resultHandler(event:ResultEvent):void
        {
            tutortable.dataProvider=getAllStaff.lastResult;
            staffnames = new Array();
            for each (var i:Staff in getAllStaff.lastResult) 
            { 
                staffnames.push(i.name);
                staff.push(i);

            }   

        }

错误：

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mx.controls::DataGrid/itemEditorItemEditEndHandler()[E:\dev\hero_private\frameworks\projects\mx\src\mx\controls\DataGrid.as:5321]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\hero_private\frameworks\projects\framework\src\mx\core\UIComponent.as:13128]
    at mx.controls::DataGrid/endEdit()[E:\dev\hero_private\frameworks\projects\mx\src\mx\controls\DataGrid.as:4522]
    at mx.controls::DataGrid/editorMouseDownHandler()[E:\dev\hero_private\frameworks\projects\mx\src\mx\controls\DataGrid.as:5083]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\hero_private\frameworks\projects\framework\src\mx\core\UIComponent.as:13128]
    at mx.managers::SystemManager/mouseEventHandler()[E:\dev\hero_private\frameworks\projects\framework\src\mx\managers\SystemManager.as:2924]