I didn't follow it that closely, but I have a couple thoughts.
Checking QueryResult == null doesn't hurt and is certainly cautious, I just can't think of any instance that will return a null QueryResult, just an empty one.
Windchill's Content stuff can be a little confusing at first. Your Representation IS the ContentHolder, so I would say defaultRep = (Representation)ContentHelper.service.getContents(defaultRep).
Most importantly, I wouldn't put this stuff directly in an expression robot/transition. Place it inside of another class and make a single call to that from your wf expression. Doing so will allow you to easily change the contents of the method if you ever need to. Off the top of my head, you may want to change logging statements and the File path, or you may come up with another solution for handling exceptions. Your wf expression can hold a try catch and route, but the other stuff should be external.