In the cloud on ChallengeStartedMessage, Spark.getData().nextPlayer appears to be null. When does this get set?
If you are writing Cloud Code for ChallengeStartedMessage, then Spark.getData().nextPlayer is expecting one of the incoming parameters to be "nextPlayer". You can find a list of the default ChallengeStartedMessage parameters through our api docs. To fix this, you would need to add an additional parameter called "nextPlayer" in your client code, which may have a reference to another player.