Incomplete type is not allowed

Well when (and if) the engine crashes it will give you the name of the .cpp file and the line of code that has failed instead of the .dll file and the memory address.

I ran it with Debug Symbols. It directed me to MulticastDelegateBase.h at line 165.
The code there is:

// if locked and no object, just return
if (InvocationListLockCount > 0)
{
	return;
}