When setting a logging level back to null, the JULHelper will throw a NullPointerException. Since null is a valid level for both Logback and JUL, the helper should be changed to accept and check for null.

java.lang.NullPointerException
at ch.qos.logback.classic.jul.JULHelper.asJULLevel(JULHelper.java:35)
at ch.qos.logback.classic.jul.LevelChangePropagator.propagate(LevelChangePropagator.java:66)
at ch.qos.logback.classic.jul.LevelChangePropagator.onLevelChange(LevelChangePropagator.java:57)
at ch.qos.logback.classic.LoggerContext.fireOnLevelChange(LoggerContext.java:317)
at ch.qos.logback.classic.Logger.setLevel(Logger.java:183)

See the attached patch file for a test case and fix