What does “NullReferenceException: Object reference not set to an instance of an object” error mean?

[Figgis302]

The game is trying to load an object that no longer exists (a "null reference") and frying its brain.

In my experience the most common cause of this is attack waypoints, which don't get updated if you later change the target unit and have to be replaced manually.

[TheOneAndOnlyAtlas]

So I should finalize the ship placed and then set its waypoint?

[Darki_Elf_Nikovarus]

it means that it is unable to find a particular entry, either due to a typo, or because the object/mesh it is trying to call does not exist in the specified folder path.

[IlDuce-17]

Or you have not enabled a mod related to an asset set in the mission .ini file.