[Bug] Battlecry crash in Sandbox Mode

[demilich1]

Clicking "Stop" while being prompted to select a Battlecry target in Sandbox Mode crashes the program.

[akoscz]

Here's a stacktrace of this crash

22:00:05 ERROR - ERROR while playing card [MINION 'Abusive Sergeant' Manacost:1] reference: [CardReference playerId:0 cardName:Abusive Sergeant cardLocation:HAND cardId:70]
22:00:05 ERROR - =========PANIC DUMP=========
java.lang.NullPointerException
    at net.demilich.metastone.game.logic.GameLogic.performGameAction(GameLogic.java:1174)
    at net.demilich.metastone.game.logic.GameLogic.resolveBattlecry(GameLogic.java:1468)
    at net.demilich.metastone.game.logic.GameLogic.summon(GameLogic.java:1615)
    at net.demilich.metastone.game.actions.PlayMinionCardAction.play(PlayMinionCardAction.java:41)
    at net.demilich.metastone.game.actions.PlayCardAction.execute(PlayCardAction.java:44)
    at net.demilich.metastone.game.logic.GameLogic.performGameAction(GameLogic.java:1183)
    at net.demilich.metastone.game.GameContext.performAction(GameContext.java:327)
    at net.demilich.metastone.game.GameContext.playTurn(GameContext.java:366)
    at net.demilich.metastone.game.GameContext.play(GameContext.java:336)
    at net.demilich.metastone.gui.sandboxmode.commands.CreateNewSandboxCommand$1.run(CreateNewSandboxCommand.java:46)
    at java.lang.Thread.run(Thread.java:745)
22:00:05 INFO  - Shutting down!
:app:run FAILED