My understanding is that it is impossible to have a NULL reference. Can you show us some code that ends up with a NULL reference? I'd be curious to see how that can be done!

Dereference a NULL pointer and use the result to initialize a reference. Sure, it's undefined behavior, but so are all the problems that result from NULL pointers, and in practice it doesn't crash until you "dereference" the reference by looking at the value, so it's probably as hard to track down as a NULL pointer crash.

I'll leave it as an excercise to you. Promise me that I don't have to maintain your C++ code until you completed it.