I've been working on an issue recently. While trying to call the disable-job CLI method, we receive:
ERROR: object is not an instance of declaring class
The root cause is because a plugin has overridden the method: https://github.com/jenkinsci/multi-branch-project-plugin/blob/master/src/main/java/com/github/mjdetullio/jenkins/plugins/multibranch/TemplateDrivenMultiBranchProject.java#L480
So now, if you try to disable a Freestyle job, it fails because it's expecting a TemplateDrivenMultiBranchProject one.
I open this to have feedback about what is best to do here: