Null reference in GameMaster update.

[mklingen]

https://sentry.io/cfg/dwarfcorp/issues/725636156/

NullReferenceException: Object reference not set to an instance of an object.
  File "C:\Users\matth\Desktop\dwarfcorp\DwarfCorp\DwarfCorpXNA\Scripting\Player\GameMaster.cs", line 487, in Update
    Void Update(DwarfCorp.DwarfGame, DwarfCorp.DwarfTime)
  File "C:\Users\matth\Desktop\dwarfcorp\DwarfCorp\DwarfCorpXNA\World\WorldManager.cs", line 554, in Update
    Void Update(DwarfCorp.DwarfTime)
  File "C:\Users\matth\Desktop\dwarfcorp\DwarfCorp\DwarfCorpXNA\GameStates\PlayState.cs", line 289, in Update
    Void Update(DwarfCorp.DwarfTime)
  File "C:\Users\matth\Desktop\dwarfcorp\DwarfCorp\DwarfCorpXNA\GameStates\GameStateManager.cs", line 117, in Update
    Void Update(DwarfCorp.DwarfTime)
  File "C:\Users\matth\Desktop\dwarfcorp\DwarfCorp\DwarfCorpXNA\DwarfGame.cs", line 582, in Update
    Void Update(Microsoft.Xna.Framework.GameTime)

Object reference not set to an instance of an object.

[Blecki]

This appears to be while resetting minion position constraints. Put in some null checks that will give us more information if it happens again.