custom component reuse with extjs4-Collection of common programming errors

I have created a custom grid in a javascript file. I want to use it as a xtype on to different panels in seperate js files. If I use it on a single panel, it works fine; but when I use try it use it on different panels at the same time, I get a error in the chrome developer tool console saying:

Uncaught TypeError: Cannot read property 'isComponent' of undefined 

My grid definition is as follows:

Ext.define('DataBox.view.FootNotes', {
    extend : 'Ext.grid.Panel',
    alias : 'widget.footNotes',
initComponent : function() {
    this.columns = [ {
        header : 'SL', 
        field : {
            xtype : 'checkbox',
            checked : 'true'
        }
    }, {
        header : 'Symbol',
    }, {
        header : 'Notes', 
    }, ];
    this.callParent(arguments);
}

});

and to include the grid on the panels i use following code

initComponent : function() {
    /*  this.footNotesInfoGrid = Ext.create('DataBox.view.FootNotes', {
        colspan: 2,
        border: false,
        frame: 'false'
    }); even tried this */

    Ext.apply(this,{
        items : [{
            xtype : 'footNotes',
            columnWidth : .50,
            id : 'infoFootNotesGrid',
        }]
    }
   }

I tried many other ways suggested on different forum discussions.. but my problem stil persists. Can somebody point out where I am going wrong?