After upgrading to Jenkins 1.638 only some of our timed builds and polling builds are being triggered. Jenkins logs show:

Nov 18, 2015 10:31:00 AM WARNING hudson.triggers.Trigger$Cron doRun
Cron thread throw an exception
java.lang.NullPointerException
at hudson.model.Descriptor.toMap(Descriptor.java:887)
at hudson.util.DescribableList.toMap(DescribableList.java:151)
at hudson.model.AbstractProject.getTriggers(AbstractProject.java:1610)
at hudson.triggers.Trigger.checkTriggers(Trigger.java:265)
at hudson.triggers.Trigger$Cron.doRun(Trigger.java:221)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:50)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)