Previous URLtrigger Plugin version installed: v0.41. Several jobs defined using "Poll with a URL" work for months until the URLtrigger v0.43 is applied. Jobs attempt to poll on normal (cron) scedule, but in the URLtrigger log I get error, "[ERROR] - Polling error..." and in the Jenkins server log, I get the following.

Jun 12, 2018 11:13:00 AM org.jenkinsci.lib.xtrigger.AbstractTrigger reportError
WARNING: Polling failed
java.lang.NullPointerException
        at org.jenkinsci.plugins.urltrigger.URLTrigger.checkIfModifiedEntryForHttpOrHttpsURL(URLTrigger.java:262)
        at org.jenkinsci.plugins.urltrigger.URLTrigger.checkIfModifiedEntry(URLTrigger.java:248)
        at org.jenkinsci.plugins.urltrigger.URLTrigger.checkIfModified(URLTrigger.java:225)
        at org.jenkinsci.lib.xtrigger.AbstractTrigger$Runner.run(AbstractTrigger.java:202)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:748)

Polling was defined to examine a JSON file and inspect the JSONpath ".BuildNum". Manually editing the job definition to remove and then identically re-define the URL spec makes the polling work again.