i am using superboxselect in grid inside a row editor in column model.

while editing the row when individual item is removed from superboxselect first time it works fine second time it gives me an error saying

"
Message: 'this.dom' is null or not an object
Line: 5465
Char: 10
Code: 0
URI: http://localhost:8094/GA-Working/ext...t-all-debug.js
"

can any one pls help its critical error in my project.....

Thanx in advance.........

here is some code i have :
Code:
var ioKRAColumns = new Ext.grid.ColumnModel({
    columns: [
    // new Ext.grid.RowNumberer(),
    {
        dataIndex : 'function_ids',
        hidden: true,
        hideable:false//User cant make this Column visible
    },
    {
        header : "Function Involved",
        width: 350,
        height:250,
        sortable : true,
        dataIndex : 'function_names',
        //editor: addKRA_functions_superbox
        editor:{
            xtype:'superboxselect',
            id:'kra-functions-superbox',
            //lazyRender: true,//should always be true for editor By enabling lazyRender this prevents the combo box from rendering until requested
            resizable: true,
            anchor:'100%',
            mode: 'local',
            forceFit: true,
            store:functionComboStore,
            displayField: 'combovalue',
            displayFieldTpl: '{combovalue}',
            valueField: 'comboid',
            triggerAction: 'all',
            typeAhead: true,
            forceSelection :true,
            allowBlank : false
        }
    },{
        dataIndex : 'imperative_category_id',
        hidden: true,
        hideable:false//User cant make this Column visible
    },{
        header : "Categorization",
        width : 200,
        sortable : true,
        dataIndex : 'imperative_category_name',
        editor: new Ext.form.ComboBox({
            id:'kra-imp-cat-combo',
            emptyText:"Select Category",
            store:imperativeCatComboStore,            
            displayField: 'combovalue',
            valueField: 'comboid',
            triggerAction: 'all',
            forceSelection :true,
            allowBlank : KRA_CATEGORY_ALLOW_BLANK
        })
    },{
        dataIndex : 'kra_id',
        hidden: true,
        hideable:false//User cant make this Column visible
    },{
        header : "KRA",
        width : 150,
        sortable : true,
        dataIndex : 'kra_name',
        editor: new Ext.form.TextArea({
            allowBlank : false
        })
    },{
        dataIndex : 'initiative_id',
        hidden: true,
        hideable:false//User cant make this Column visible
    },{
        header : "Measure of Success",
        width : 150,
        sortable : true,
        dataIndex : 'measure_of_success',
        editor: new Ext.form.TextArea({
            allowBlank : false
        })
    },{
        header : "Milestone",
        width : 150,
        sortable : true,
        dataIndex:'milestone',
        editor: new Ext.form.TextArea({
            allowBlank : true
        })
    },{
        header : "Assigned To",
        width : 150,
        sortable : true,
        //dataIndex:'milestone',
        dataIndex : 'assignee_user_names',
        editor: new Ext.form.TextArea({
            allowBlank : false
        })
    },{
        header : "Remarks",
        width : 150,
        sortable : true,
        dataIndex : 'remarks',//
        editor: new Ext.form.TextArea({
            ///allowBlank : false
            })
    },{
        header : "Actual Users",
        width : 350,
        sortable : true,
        dataIndex : 'actual_user_names',//
        hidden:ACTUALUSERVISIBLE,
        hideable:false,//User cant make this Column visible
        editor: new Ext.ux.form.SuperBoxSelect({
            id:'kra-actual-user-names',
            resizable: true,
            // store: userComboStore,
            store:kraAssigneeUserComboStore,
            mode: 'local',
            displayField: 'combovalue',
            displayFieldTpl: '{combovalue}',
            valueField: 'comboid',
            // queryDelay: 0,
            triggerAction: 'all',
            allowBlank:KRA_CATEGORY_ALLOW_BLANK
        })
    },{
        header : "Actual User IDs",
        dataIndex : 'actual_user_ids',
        hidden: true,
        hideable:false//User cant make this Column visible
    }
    ]