NRE in DeliverUnit.ReleaseUnit after player surrendered

[pchote]

Reported by user Evulant on the community Discord:

OpenRA engine version release-20210321
Dune 2000 mod version release-20210321
on map 3f0adb12512918b6f6a0a6666c117b750c589e3d (The Great Flat by Westwood Studios).
Date: 2021-03-24 22:53:21Z
Operating System: Windows (Microsoft Windows NT 6.1.7601 Service Pack 1)
Runtime Version: .NET CLR 4.0.30319.42000
Exception of type `System.NullReferenceException`: Object reference not set to an instance of an object.
   at OpenRA.Mods.Common.Activities.DeliverUnit.ReleaseUnit.OnFirstRun(Actor self)
   at OpenRA.Activities.Activity.TickOuter(Actor self)
   at OpenRA.Traits.ActivityUtils.RunActivity(Actor self, Activity act)
   at OpenRA.Activities.Activity.TickChild(Actor self)
   at OpenRA.Activities.Activity.TickOuter(Actor self)
   at OpenRA.Traits.ActivityUtils.RunActivity(Actor self, Activity act)
   at OpenRA.Actor.Tick()
   at OpenRA.World.Tick()
   at OpenRA.Game.InnerLogicTick(OrderManager orderManager)
   at OpenRA.Game.LogicTick()
   at OpenRA.Game.Loop()
   at OpenRA.Game.Run()
   at OpenRA.Game.InitializeAndRun(String[] args)
   at OpenRA.WindowsLauncher.WindowsLauncher.RunGame(String[] args)

d2k-2021-03-24T222912Z.orarep.zip