Go through the stack trace (that appears not to be the complete stack trace) and work out what could be null. Look at the highest line in the stack trace in the code you wrote, and check every object reference in that for a null. Check you have initialised every object reference in your constructor or elsewhere. Check you aren't assigning anything to null mistakenly, or passing anything as an argument which could be null. Check you really have those .mdi files and they are complete.
If you don't find anything hopeful, please come back with more details.