100% way to crash Xtreme (NULL pointer)

[instinctive.de]

A null pointer exception in general most probably means that Xtreme tried to access an object (image, vector shape, menu button, window, view, ...) that does not exist (anymore).

In this case, the explanation is probably that the substract operation never actually took place (which is actually reflected in its visible result), and then Xtreme tries to "undo" it

[handrawn]

Thanks