Please use copy‑and‑paste for such messages.
Try javac -Xlint:unchecked and compile it again. You can find out about the lint tool here and here.
That particular warning (not error) probably means that you are using a parametrised type (probably the Combo Box) without giving it an actual type parameter. If you add an actual type parameter to the combo box declaration and don’t use a raw type, you can be sure it will work.
Another way to put it: you ought to use generics somewhere and you didn’t. If you find old versions of the combo box API you can see that only the most recent versions of combo box actually had a formal type parameter. You can read about generics in the Java Tutorials.