如何从文档侦听器更新JComboBox的列表？

I'm writing a custom JComboBox and whenever the user types something I want to update the drop down menu of my JComboBox. The issue I'm having is that once my DocumentListener sees an update I get an error when I try to add an item to the list. Here's a basic example of what isn't working:I'm writing a custom JComboBox and whenever the