It seems to be a bug in dispose method, which is solved in master:
https://github.com/droolsjbpm/jbpm/blob/5.1.0.M1.x/jbpm-bam/src/main/java/org/jbpm/process/audit/JPAProcessInstanceDbLog.java
https://github.com/droolsjbpm/jbpm/blob/master/jbpm-bam/src/main/java/org/jbpm/process/audit/JPAProcessInstanceDbLog.java
em is null because you never used it.
Anyway, to use this Process instance Log,
you can just use processInstaceLogger.find<Process/Node/Variable>Instances method to get the information you need.