Task#isWorking sometimes throws

[MadaraUchiha]

Issue summary

Task#(get) isWorking sometimes throws TypeErrors.

Description of issue:

Sometimes, presumably at the same tick a creep dies, an error is thrown in Task#isWorking that you cannot access pos of undefined. Happens to me for the GoToRoom task but I assume it could happen for others as well.

Full error below.

Steps to reproduce:

Just run a new colony for a while. They pop up often.

Error message:

TypeError: Cannot read property 'pos' of undefined
    at TaskGoToRoom.get isWorking [as isWorking] (main:3700:27)
    at TaskGoToRoom.run (main:3683:18)
    at Zerg.run (main:19374:30)
    at RandomWalkerScoutOverlord.autoRun (main:7107:19)
    at RandomWalkerScoutOverlord.run (main:14152:14)
    at try (main:23946:41)
    at Overseer.try (main:23664:17)
    at Overseer.run (main:23946:25)
    at _Overmind.run (main:24045:7133)
    at main (main:24298:14): Cannot read property 'pos' of undefined

// Offending line: main:3700:27
return this.creep.pos.inRangeToPos(this.targetPos, this.settings.targetRange) && !this.creep.pos.isEdge;

Suggested fix (optional):

Ensure that this.creep is not undefined prior to accessing its position. There's an assumption that it would always be there that is apparently false.

Other information:

• Overmind version: {found in package.json or by typing info() into console}

  ‣ Version: Overmind v0.5.2
  ‣ Checksum: 0x34e6838c39d
  ‣ Assimilated: No (clearance code: null) [WIP]
  ‣ Operating mode: automatic

• Commit hash: {optional, but helpful: include the commit hash (if known)}

  Taken from the 0.5.2 Release page. (So I'm guessing the commit which is tagged 0.5.2)

[MadaraUchiha]

It's worth noting that this reproduces on the private server that you spawn from npx screeps start, doesn't seem to happen on the public server.