How to add an empty item to ExtJS combobox?

Question

I want to add and empty item (display value is blank, item height is kept as normal) to an Ext.form.ComboBox. I refered 2 links below to configure my combobox, but it still

Answer 1

This is how we can add a blank field in Combo box

In java Map or any other collection put key value like this

fuelMap.put(""," "); // we need to add " " not ""," " or null 
                          // because these will add a fine blank line in Combobox 
                          // which will be hardly noticeable.

In js file, it should be like this :

Listener for combo box

listeners: {
    select: function (comp, record, index) {
        if (comp.getValue() === "" || comp.getValue() === " ") {
            comp.setValue(null);
        }
    }
}