This shows up a fair amount in issue reports, either in the CLI output or in the server log:

"failure-description" => "JBAS014749: Operation handler failed: null"

The null happens because the message is getting passed the result of throwable.getLocalizedMessage() and that may be null. See DMR-11 for what's likely the most common reason it's null.

If it's null, the code should use the class name of the error message.