I emailed IntelliJ support and got the solution. I was using the
following:

Vector fileList = new Vector(Arrays.asList(original.list()));
fileList.retainAll(Arrays.asList(translated.list()));
fileList.trimToSize();
listAdded = new JList(fileList);
this.getContentPane().setLayout(new FlowLayout());
this.getContentPane().add(this.listAdded);

There was no need to initialise the JList as its done already in the
GUI designer so the code  should have been,

Vector fileList = new Vector(Arrays.asList(original.list()));
fileList.retainAll(Arrays.asList(translated.list()));
fileList.trimToSize();
listAdded.setListData(fileList);