Dead units can enter transports (Crashes when unloaded)

[cjshmyr]

There's a tight race condition that is allowing units to die and enter transports on the same tick. This will cause the game to crash when that transport tries to unload.

Engine version: release-20190314

Replay: OpenRA-2019-04-12T191227Z.zip - happens just before 14:00, replay is 14:04 long.

Stacktrace:

OpenRA engine version release-20190314
Tiberian Dawn mod version release-20190314
on map 1823885808927deb9fbb4c746ea9e8693fd6428e (Renegade 2D (C&C 0.93) by hamb).
Date: 2019-04-12 19:26:30Z
Operating System: Windows (Microsoft Windows NT 6.2.9200.0)
Runtime Version: .NET CLR 4.0.30319.42000
Exception of type `System.InvalidOperationException`: Attempted to get trait from destroyed object (e6 244 (not in world))
   в OpenRA.TraitDictionary.CheckDestroyed(Actor actor)
   в OpenRA.TraitDictionary.Get[T](Actor actor)
   в OpenRA.Actor.Trait[T]()
   в OpenRA.Mods.Common.Traits.Cargo.<CanUnload>c__AnonStorey2.<>m__0(Actor p)
   в System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
   в OpenRA.Mods.Common.Traits.Cargo.<CanUnload>m__4(CPos c)
   в System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
   в OpenRA.Mods.Common.Traits.Cargo.CanUnload()
   в OpenRA.Mods.Common.Traits.Cargo.ResolveOrder(Actor self, Order order)
   в OpenRA.Network.UnitOrders.ProcessOrder(OrderManager orderManager, World world, Int32 clientId, Order order)
   в OpenRA.Network.OrderManager.Tick()
   в OpenRA.Game.InnerLogicTick(OrderManager orderManager)
   в OpenRA.Game.LogicTick()
   в OpenRA.Game.Loop()
   в OpenRA.Game.Run()
   в OpenRA.Game.InitializeAndRun(String[] args)
   в OpenRA.WindowsLauncher.RunGame(String[] args)

We can confirm this happens by adding an a.IsDead check to the beginning of Cargo.Load(self, a). I'm not sure of the best way to fix this though.

[abcdefg30]

Sounds like #16295 might have fixed the crash here. An IsDead check before loading wouldn't hurt, though.