This:
https://github.com/jenkinsci/join-plugin/commit/4c5fefa05cbf2f0ae26ad56bc1510e0135fb8a75#diff-600376dffeb79835ede4a0b285078036R39

… is a lie:
https://github.com/jenkinsci/join-plugin/blob/e8b1c0023f0d8e8e56aadbd7189f6be4dc6b80e0/src/main/java/join/JoinTrigger.java#L85

Not having Matrix Project installed when installing Join Plugin will result in severe failures.