Validate pointers before use - review code and fix

While doing integration testing, I came to notice that one of the key probloems causing the runtime crashes is the use of pointers without validating.

It is a must that we init all pointers to NULL and also test the pointers before use to make sure they have valid content.
This was also mentioned in coding convention.

I think we have to review the code and fix, else the engine would crash unexpectedly and it would be hard to figure out the causes.