Notebook: Remove null check after dereference

Notebook: Remove null check after dereference

This function clearly assumes the parameter children cannot be NULL, and
the call sites seem to perform enough checks to confirm this.

CID 1388869 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking children suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.