Null Ptr in MCTS game

I enabled MCTS ai, got this stack trace:
1) start game
2) choose yourself to go first
3) play a land
4) press spacebar
5) while AI is thinking press F4

Game exception occurred: java.lang.NullPointerException
Server version: 1.4.42V6 (build: 2020-03-04 17:19)
mage.player.ai.ComputerPlayerMCTS.calculateActions(ComputerPlayerMCTS.java:98)
mage.player.ai.ComputerPlayerMCTS.getNextAction(ComputerPlayerMCTS.java:112)
mage.player.ai.ComputerPlayerMCTS.priority(ComputerPlayerMCTS.java:77)
mage.game.GameImpl.playPriority(GameImpl.java:1292)
mage.game.turn.Step.priority(Step.java:61)
mage.game.turn.Phase.playStep(Phase.java:184)
mage.game.turn.Phase.play(Phase.java:89)
mage.game.turn.Turn.play(Turn.java:126)
mage.game.GameImpl.playTurn(GameImpl.java:889)
mage.game.GameImpl.play(GameImpl.java:798)
mage.game.GameImpl.start(GameImpl.java:762)
mage.server.game.GameWorker.call(GameWorker.java:34)
java.util.concurrent.FutureTask.run(FutureTask.java:266)
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
java.lang.Thread.run(Thread.java:748)

Start with mage.player.ai.ComputerPlayerMCTS.java at lines 98, 112, and 77, then follow the priority call from mage.game.GameImpl. Reproduce the listed MCTS sequence, including pressing F4 while the AI is thinking. Done means the sequence no longer raises the reported NullPointerException.

Written by the indexing model from the issue text.