viruschecker | A free antivirus aggregation software like VirusTotal | Testing library

I'm writing a program with some methods. In one of the methods I want to add some Strings to a ComboBox. This doesn't work, I can't remove or add items. But, I don't get an error either.

The method is reached well, when I print something in this method, it's in the output. Probably something is wrong with the declaration/instantiation of the combobox, but I don't know what to change. I tried public void vulDropdowns(JComboBox Host1Dropdown){ instead of just public void vulDropdowns(){, but that doesn't work. Then I get an error. It's probably something really stupid but I don't see it, and I don't have much experience with Java. It's a big program if I should put all code of all classes here. But the most important parts (I think) are:

Class VirusGUI: The ComboBox is created by the 'design' option, so it should be right, I can't edit that part of the code. When the button is pressed, the program reads a file, and some data should go to the combobox. The method readfile.choosefile is called.