This is a relatively simple fix.  I've tested it and it does fix at
least part of the problem but there is still a problem.

It is possible to grab a class that you don't have selected and drag it.
 The dragged object is labelled 'nil' and when you drop it on a category
you get a debugger.

Perhaps this is an unrelated problem and is instead a general drag and
drop problem.  When you click and drag something shouldn't it be
selected before dragging starts?

Ken