Error trying to extend ExtJs ComboBox - "Cannot read property 'dom' of null"?

Question

I'm using ExtJs4 and I'm trying to extend the Ext.form.field.ComboBox like below:

Ext.define('GenericCombo', {
  extend: 'Ext.form.field.ComboBox',
  alias: 'widget.genericcombo',

  //the constructor
  constructor: function(config) {

    var store = new Ext.data.JsonStore({
        fields: ['Key', 'Value'],
        data : config.combodata || []
    });//new Ext.data.Store

    Ext.apply(this, config, {
        store: store,
        displayField: 'Value',
        valueField: 'Key',
        queryMode: 'local',
        emptyText:'Select a value...'
    });

    this.callParent([this]);

  }//end constructor

});//end Ext.define

The data for the store i.e. config.combodata is returned in JSON format like below:

"combodata":[
            {"Key":"","Value":"<None>"},
            {"Key":"!#","Value":"Dr"},
            {"Key":"!$","Value":"Miss"}
        ]

However I get an error on line 61312 of ext-all-debug.
(inside the renderActiveError method).

Error: Uncaught TypeError: Cannot read property 'dom' of null

Line 61312 :

me.errorEl.dom.innerHTML = activeError;

Am I missing something obvious here?

EDIT: Adding some code where I instantiate it:
I actually instantiate the combobox dynamically i.e. The server returns some extjs code dynamically in JSON format like below:

 {
    "anchor":"50%",
    "autoScroll":false,
    "border":false,
    "combodata":[
          {"Key":"","Value":"<None>"},
          {"Key":"!#","Value":"Dr"}
        ],
    "fieldLabel":"Title",
    "name":"3820",
    "value":"!)",
    "xtype":"genericcombo"
 }

However When i try to hardcode it i get the same error. Hardcoded example:

            xtype: 'form',
            title: 'A Form',
            items:[{
                     xtype: 'genericcombo',
                     fieldLabel: 'Test',
                     combodata: [{Key: 'one', Value: 'two'}]
                  }]

Solution 1

I was calling

this.callParent([this]); //Which is wrong and caused my error.

The correct way is to call

this.callParent([arguments]);

Solution 2

Try this... move everything in your constructor to the initComponent method. Then in your constructor you need to call the parent's constructor...

constructor : function(config) {
   GenericCombo.superclass.constructor.apply(this,new Array(config);
}

I would also consider namespacing your component... something like Ext.ux.GenericCombo would be better suited.