[dev] Exception resolving battle with damaged/destroyed dropship in opfor

[NickAragua]

When resolving a battle that involves a dropship with a blown-out bay, we get the following exception.

java.lang.NullPointerException
at mekhq.campaign.parts.BayDoor.remove(BayDoor.java:104)
at mekhq.campaign.parts.TransportBayPart.updateConditionFromEntity(TransportBayPart.java:110)
at mekhq.campaign.unit.Unit.runDiagnostic(Unit.java:536)
at mekhq.campaign.unit.TestUnit.(TestUnit.java:42)
at mekhq.campaign.ResolveScenarioTracker.generateNewTestUnit(ResolveScenarioTracker.java:162)
at mekhq.campaign.ResolveScenarioTracker.processGame(ResolveScenarioTracker.java:386)
at mekhq.MekHQ.gameVictory(MekHQ.java:577)
at megamek.common.event.GameVictoryEvent.fireEvent(GameVictoryEvent.java:70)
at megamek.common.Game.processGameEvent(Game.java:3161)
at megamek.client.Client.handlePacket(Client.java:1545)
at megamek.client.Client$1$2.run(Client.java:193)

Clicking the 'ok' button multiple times when using 'resolve manually' with a MUL resulting from the scenario for some reason eventually lets the scenario resolution proceed.

Example campaign. Use the attached MUL file to resolve "Dropship Assault".
1st Octopus Overlords.zip

[sixlettervariables]

The order of these two lines is incorrect in BayDoor:

            parentPart.removeChildPart(this); // sets parentPart to null.
            parentPart.addChildPart(missing);

Also messed up in Cubicle::remove.

[sixlettervariables]

I'll fix that as part of my current PR to fix the stack overflow.