When ever i am removing the selected text of combo box and and moving to the other column of editable gird via MOUSE the it work fine and set blank text to field but

when ever i am navigating other field via TAB KEY then is store the last selected value rather than the blank.

{
header: "Settlement Tender",

sortable: false,
width:25 ,
lazyRender : false,
dataIndex: 'settlementTender',
editor: new Ext.form.ComboBox({
id:'id_SettlementTender',
triggerAction: 'all',
emptyText: 'Select Field...',
selectOnFocus:true,
fieldLabel: ' ',
labelSeparator: ' ',
editable: true,
forceSelection: true,
allowBlank: true,
beforeBlur: function(){

var value = this.getRawValue();
if(value == ''){
this.reset();
}
},
valueField: 'bankingLineId',
displayField: 'bankingLineId',
store: new Ext.data.Store({
proxy: new Ext.data.HttpProxy({
url: URL
}),
reader: new Ext.data.JsonReader(
{ root: 'results', totalProperty: 'totalCount' },
['recNo', 'bankingLineId', 'description', 'functionType']
),
sortInfo: { field: 'recNo', direction: 'asc' },
//baseParams: { 'groupId': utilsObj.groupId, 'status': 1 },
remoteSort: true
})
})}