This looks to be introduced after 8245: https://github.com/openjdk/jmc/commit/5016cc1919616f559cec95a20dc3ae2edb721dc3

I'm seeing a bunch of warnings and errors logged when just navigating JMC and doing basic functionality. They don't seem to be inhibiting any actions, but would be worthwhile to look into. They logs look something like:

[..]
Warning: validation was turned on but an org.xml.sax.ErrorHandler was not set, which is probably not what is desired.  Parser will use a default ErrorHandler to print the first 0  errors.  Please call the setErrorHandler method to fix this.
Error: URI=null Line=1: Document root element "state", must match DOCTYPE root "null".
Error: URI=null Line=1: Document is invalid: no grammar found.
Warning: validation was turned on but an org.xml.sax.ErrorHandler was not set, which is probably not what is desired.  Parser will use a default ErrorHandler to print the first 0  errors.  Please call the setErrorHandler method to fix this.
Error: URI=null Line=44: Document root element "RJMX", must match DOCTYPE root "null".
Error: URI=null Line=44: Document is invalid: no grammar found.
Warning: validation was turned on but an org.xml.sax.ErrorHandler was not set, which is probably not what is desired.  Parser will use a default ErrorHandler to print the first 0  errors.  Please call the setErrorHandler method to fix this.
Error: URI=null Line=1: Document root element "server_model", must match DOCTYPE root "null".
Error: URI=null Line=1: Document is invalid: no grammar found.
Warning: validation was turned on but an org.xml.sax.ErrorHandler was not set, which is probably not what is desired.  Parser will use a default ErrorHandler to print the first 0  errors.  Please call the setErrorHandler method to fix this.
Error: URI=null Line=1: Document root element "triggers", must match DOCTYPE root "null".
[..] etc.