Ext.getCmp(id) undefined

Hello experts,

I am developing an application and have two grids as A and B, on click of an action button in grid B, I am trying to change the action column icon of particular row in grid A. I have tried it as follows

In handler of grid B;s action column,

var grid = Ext.getCmp('stockgrid'); //stockgrid is Grid A's Id

var button = Ext.query('td.x-action-col-cell img', grid.getNode(index))[2];

button.src = '.../add.png';

But I am gettingExt.getCmp('stockgrid'); asundefined.

Please help

Ext.getCmp(id) undefined