This class must implements Describable 

This class must implement Describable otherwise Fitnesse -sometime- can't launch the plugin

I have the exception: 
java.lang.AssertionError: class hudson.plugins.fitnesse.FitnesseBuilder is missing its descriptor
        at jenkins.model.Jenkins.getDescriptorOrDie(Jenkins.java:1165)
        at hudson.tasks.Builder.getDescriptor(Builder.java:67)
        at hudson.plugins.fitnesse.FitnesseBuilder.getDescriptor(FitnesseBuilder.java:278)

See https://wiki.jenkins-ci.org/display/JENKINS/My+class+is+missing+descriptor