If I configure my job to use external resource selection criteria without first having reconfigured the Jenkins url I get the error below (also see attachment).
In other words the url under Manage Jenkins/
Configure System/Jenkins url is set to http://localhost:8080 when this error occurs. If I change the url to something else (say http://test:8080) the error doesn't occur.

The effect is that all build threads on the current slave server die with this error message:

Exception in thread "Executor #1 for master" java.lang.IllegalStateException: Root URL isn't configured yet. Cannot compute absolute URL.
at hudson.model.AbstractItem.getAbsoluteUrl(AbstractItem.java:419)
at com.sonyericsson.jenkins.plugins.externalresource.dispatcher.ExternalResourceQueueTaskDispatcher.getUrl(ExternalResourceQueueTaskDispatcher.java:176)
at com.sonyericsson.jenkins.plugins.externalresource.dispatcher.ExternalResourceQueueTaskDispatcher.canTake(ExternalResourceQueueTaskDispatcher.java:122)
at hudson.model.Queue$JobOffer.canTake(Queue.java:254)
at hudson.model.Queue.maintain(Queue.java:1032)
at hudson.model.Queue.pop(Queue.java:863)
at hudson.model.Executor.grabJob(Executor.java:285)
at hudson.model.Executor.run(Executor.java:206)