And the debug call stack:
#0 00424842 SearchCell::get_f(this=0x0) (C:/Projects/Individuellt Mjukvaruutvecklingsprojekt/Game/DungeonsOfZiro/files/SearchCell.h:33)
Well, there's your problem. You're invoking the get_f() method on a null pointer.
EDIT: This means one of your operands in a call to bool cell_cmp::operator()(const std::shared_ptr<SearchCell>& a, const std::shared_ptr<SearchCell>& b) const is an empty shared_ptr.