This happens when someone (or something) tries to download from http://<yourserver>/file, the download fails for whatever reason and you don't specifiy a ?target=blabla parameter. This is because it tries to create a 404 iframe, but doesn't know where to place it.
I'll file a patch to fix the nullpointer, but I can't tell you why you're getting these requests.
I've seen similar messages and thought they appeared because I had a browser with a session open that sends some http-request whilst the modeler has restarted.