Fix a crash when change a material reflect cubemap.

Fix for  #694 Cubemap R6025 

This is the problem of allow pointers and smart pointers on same code.

What happens:

* We have a pointer to object
* We assign pointer to StrongPtr
* Bla, bla bla bla...
* Clean StrongPointer -> 0 refs -> delete object... oh no!!! :(
* We use our first pointer... pointing to a deleted object :(