"Obviously a null pointer error."
How is this obvious? I can't draw this conclusion from the posted stacktrace.
Seen this before...
In all cases, there'd be a critical error somewhere earlier in the log (stackoverflow, OutOfMemory, GCOverhead). This'd bring down the akka actorsystem that Mendix runs on, so that any subsequent action would result in a stacktrace such as yours. Basically it's a consequence, not a cause.
Given that you also indicate your application stops working, I'd guess you have the same problem. Try and run through your log to find the actual culprit...