Fix #29472 - nullptr while trying to find a NPC in the overmap

[neitsa]

Summary

SUMMARY: Bugfixes "Fix #29472 - nullptr while trying to find a NCP in the overmap"

Purpose of change

Fix #29472 - nullptr while trying to find a NCP in the overmap

There were multiple missing checks on find_npc method which might return nullptr if the NCP can't be found in the overmap.

Describe the solution

• Add the missing check for the crash issue.
• Check all other locations where the function is used.

Add a simple check for nullptr. Add some debug msgs in missions to better diagnose the missing NPC problem.

Describe alternatives you've considered

There are probably no simpler alternatives.

Additional context

N/A.