ERRORHANDLER internal error: Error while waiting for activity completion:null

[msmygit]

Describe the bug
When a scenario errors, we get this at the very end,

4518 INFO  [main] NBCLI        executions: 1 scenarios, 0 normal, 1 errored
    4518 WARN  [main] NBCLI        executions: 1 scenarios, 0 normal, 1 errored
    4521 ERROR [main] ERRORHANDLER internal error: Error while waiting for activity completion:null
java.lang.RuntimeException: Error while waiting for activity completion:null
	at io.nosqlbench.engine.core.lifecycle.activity.ActivityExecutor.awaitActivityCompletion(ActivityExecutor.java:510)
	at io.nosqlbench.engine.core.lifecycle.activity.ActivityExecutor.call(ActivityExecutor.java:420)
	at io.nosqlbench.engine.core.lifecycle.activity.ActivityExecutor.call(ActivityExecutor.java:60)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:833)
Error while waiting for activity completion:null

Additional context
Add any other context about the problem here.
OS: Linux (ubuntu 18.04)
version info (15.17.5-preview)