Dan,
The error is from CodeMix. CodeMix will not work with Eclipse 2021-12. Unfortunately we have no workaround to suggest, other than using a prior Eclipse version (Eclipse 2021-09 or earlier with Java 11).
Please note that CodeMix has reached its end of life and MyEclipse 2021 is the alternative. See here for more details : https://www.genuitec.com/products/codemix/end-of-life-notice/
If you are already a licensed CodeMix user, then we can convert your license to work with MyEclipse at no additional cost.
Apologies for inconvenience caused. Please let us know if you have any further questions.
–Swapna
Genuitec Support