Reproduction:

• Check the option to use artifactory for releases
• Don't set the option to deploy to artifactory
• Do a release, but don't have the plugin create a tag, use a branch

Result NullpointerException during build. At line 89 in https://github.com/jenkinsci/artifactory-plugin/blob/master/src/main/java/org/jfrog/hudson/release/scm/git/GitCoordinator.java the tagComment will be null and because deployment is turned off, the default is also null.

Workaround for this issue is therefore to always use a tag for releasing.