This is very hard to reproduce, but appeared on Jenkins with C* 2.1 (I've only seen it once), so here it is.
The started_at cell value from the system_traces.sessions table can be null, after duration is not null or at least that is what is shown on the exception (mono):
Adding more info:
Just saw this error in a nightly build today (February 18th). It happened in the following test running against dse-4.8 with netstandard2.0 target: