Populate ComboBox

Hi guys,
What method do I used to eliminate the null or automatically populate my combobox with values? I have 3 combobox that listen to changes.

ex. Company, Trade, Branch. Each time I select a company it will populate the list of trades from selected company and if I select a trade it will populate the list of branch from the selected trade. now, the problem occurs when I select a company since it throw a null values from the branch. so how do I populate the trade without those null values? I want the values in the trade combobox to appear automatically each time I select the company. comboBox.setNullSelectionAllowed(false) doesnt work.