[CLOSED] Set Combo Box selected item on page load

[registrator]

For my combo box

X.ComboBoxFor(m => m.TYPE).ID("ComboBoxType").MultiSelect(false).LabelWidth(50).Width(150).Editable(false).Items(new ListItem("Invoice", "INV"), new ListItem("Pro-Forma", "PRO")).SelectedItems(X.ListItem().Value("INV")),

I would like to select the first item on opening the window where the combo box is.

Ext.onReady(function() {
            var combo = Ext.getCmp('ComboBoxType');
            combo.setValue('INV');
        });

I keep getting

Uncaught TypeError: Cannot read property 'setValue' of undefined

What am I doing wrong please?