Reported in JENKINS-26157 but this is a bug in the Subversion plugin. After a SVNCancelException (failure to check out) from UpdateUpdater$TaskImpl.perform, null is returned, in violation of the documented contract of UpdateTask.perform. Eventually this gets passed up to checkout, which tries to add that to externals and fails.

perform should either return an empty list, or more likely just throw something appropriate such as AbortException. Was broken a couple years ago: https://github.com/jenkinsci/subversion-plugin/commit/7155ad9