ExtJS 6.5 - SelectionModel select not working

Question

I am trying to select the first row in a grid:

Ext.getCmp('myGrid').getSelectionModel.select(0);

I got an error saying select is undefined

but I can see the function exists.

I was using the ExtJS 4.2 version and I had no problem with that, now I am using the 6.5 version and I got that error.

What could be the problem here?

--EDIT--

Basically I have a function to create the store:

CreateDataStore: function (result, id) {
    Ext.define('DDM', {
        extend: 'Ext.data.Model',
        fields: result.FieldColumns
    });

    var dataRecords = JSON.parse(result.Records);

    return Ext.create('Ext.data.Store', {
        id: id,
        autoLoad: true,
        model: 'DDM',
        data: dataRecords,
        remoteSort: false,
        fields: result.Fields
    });
}

And another function to assign the store to the grid and select the first row:

function BindDataGrid(result) {
 gridStore = CreateDataStore(result, 'myGridStore');
 Ext.getCmp('myGrid').reconfigure(gridStore);
 if (Ext.getCmp('myGrid').getStore().getCount() > 0)             
  Ext.getCmp('myGrid').getSelectionModel().select(0);
}