Avoiding a CCE during form validation, such as when initially configuring a job with both Git SCM and Git Publisher post-build step.
…/job/…/descriptorByName/GitPublisher/checkRemote?value=origin
java.lang.ClassCastException: hudson.scm.NullSCM cannot be cast to hudson.plugins.git.GitSCM
	hudson.plugins.git.GitPublisher$DescriptorImpl.doCheckRemote(GitPublisher.java:419)