If setTimeBasedRollingPolicy has not been set on a SizeAndTimeBasedFNATP class a java.lang.NullPointerException is thrown rather than a cleaner reporting of the issue.

Exception in thread "main" java.lang.NullPointerException
at ch.qos.logback.core.rolling.TimeBasedFileNamingAndTriggeringPolicyBase.start(TimeBasedFileNamingAndTriggeringPolicyBase.java:44)
at ch.qos.logback.core.rolling.SizeAndTimeBasedFNATP.start(SizeAndTimeBasedFNATP.java:37)

The reason for this seems that within TimeBasedFileNamingAndTriggeringPolicyBase.start

The following line is executed without first checking that tbrp has been setup first.

public void start() {
DateTokenConverter dtc = tbrp.fileNamePattern.getPrimaryDateTokenConverter();
.....