Widget combobox does not have drop down selection menu in Grid

{

text: 'Run Mode',

dataIndex: 'rMode',

width: 150,

xtype: 'widgetcolumn',

widget: {

xtype: 'combo',

//value: 'Update',

store: ['a','b'],

listeners: {

'select' : function(combo){

//changeComponentValue(combo,gridModel,"rMode");

}

}

}}



The widget column combobox does not have a drop down menu when I click the combo box.



Also, this is a check tree grid, when I check any tree node, the widget column will disappear.



Am I using this in a wrong way?



Thanks