I think it is not possible to end the process in a decision handler, cause a decision is not a stable state in process. So after the process execute the code in decision, the process try to take a transition and throw the exception while check if the process is still active.
Please note that a process should always end in an end activity.
Maybe you can show your process. I created processes with similar behavior too. In my case I ended concurrent processes in a java activity and synchronzied with a join activty.