When the different levels are switching in my demo,the engine sometimes tells me
that ... pointer is empty.So I let the engine wait for some frames when the levels are switching,but if there are many pointers in the levels,how could I do it well?

If you use a pointer, make sure that it is filled:

while(pointer == NULL){wait(1);}

or even shorter:

while(!pointer){wait(1);}

This will pause the function until the pointer is valid.

Yes,you are right.I have used the same method: while(pointer == NULL){wait(1);}
it does well.Thank you.