A property (prop) on an object is realized by a getter (ie getProp()). The getter throws a NullPointerException on invokation.
Trying to display the property, the propertydisplay component reports an error "Property 'currentlyOverdue' contains a null value in the path." without any trace of the underlying NPE.
The error report should include the stack-trace off the NPE and the error message could also be a bit clearer.
It's possible that propertydisplay also fails to report other exceptions that occur in a getter.