3.4 throws "null" exception on grid template loading.
Working perfectly with 3.2.0

I am loading a grid from a JSON reader based Store with help of MemoryProxy.

It is working good in 3.2.0 but not in 3.4.
In 3.4 it gives exception in all browsers
"SCRIPT5007: Unable to get value of the property 'child': object is null or undefined
ext-all-debug.js, line 47024 character 13".

When I checked the code difference.

3.2 has
var el = this.grid.getGridEl().dom.firstChild;
var cs = el.childNodes;
this.mainWrap = new E(cs[0]);
But 3.4 has
el = Ext.get(this.grid.getGridEl().dom.firstChild),
mainWrap = new Element(el.child('div.x-grid3-viewport')),

Please help me to figure out the issue.