Very good programming.

Removed entities are not NULL. An Entity cannot be NULL. NULL is a value that can be assigned to pointers. Lite-C has no way of finding out where you saved pointers to entities and set them automatically to NULL for you. Your code crashes because myTarget is a vagabonding pointer, not because it is NULL and suddenly not NULL. A value different from NULL does not mean a pointer is necessarily valid.

http://tutorial.3dgamestudio.net/

is there an instruction to let the engine test if
a pointer is valid?

The pointer is known, the engine knows its objects internally.
So it should be a simple step to confirm the validity
of the object by iterating though the list of objects.

You can easily do that yourself by iterating over the engine objects with ptr_first (void* object). If the pointer you want to check is amongst the engine objects it is valid, otherwise it is not. However, this is not recommendable in terms of speed and won't work for other memory blocks you have allocated yourself.