REST processor diagnostics fails

[Kei Schaff]

 The NiFi 1.9.2 REST call to view the diagnostics on any processor (GET
/processors/{id}/diagnostics) in a mutlinode-node secure cluster, returns

HTTP/1.1 500 Internal Server Error
. . .
An unexpected error has occurred. Please check the logs for additional details.

Only when I set the log-level to TRACE, do I see a stack trace in the log
file, indicating that there is a NullPointerException originating at line
3537 of org.apache.nifi.web.api.dto.DtoFactory:

usageDto.setFileStoreHash(DigestUtils.sha256Hex(flowController.getProvenanceRepoFileStoreName(repoName)));

On the hunch that it is the getProvenanceRepoFileStoreName() method that is
returning the NULL, I changed nifi.provenance.repository.implementation
from org.apache.nifi.provenance.VolatileProvenanceRepository to
org.apache.nifi.provenance.WriteAheadProvenanceRepository. After making
this change, the processors diagnostics REST calls succeed.

Has anyone else seen, or can someone else duplicate, this issue?