"Could not commit session: java.lang.NullPointerException" after a migration from jBPM 5 to BPM Suite 6

Issue

We had a migration from jBPM 5 / BRMS 5 to BPM Suite 6 recently. We have several tasks in our JBPM process that are stuck "InProgress" status. When we try to complete the task it fails and the task status keeps "InProgress". The status never gets to the "Completed" state.
The error stacktrace in server.log looks like:

06:54:39,268 WARN  [org.jbpm.services.task.persistence.TaskTransactionInterceptor] (http-jboss1/192.168.1.1:8080-12) Could not commit session: java.lang.NullPointerException
    at org.jbpm.services.task.wih.ExternalTaskEventListener.afterTaskCompletedEvent(ExternalTaskEventListener.java:116) [jbpm-human-task-workitems-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.events.TaskEventSupport.fireAfterTaskCompleted(TaskEventSupport.java:211) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.internals.lifecycle.MVELLifeCycleManager.taskOperation(MVELLifeCycleManager.java:359) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.impl.TaskInstanceServiceImpl.complete(TaskInstanceServiceImpl.java:153) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:76) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:39) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompositeCommand.execute(CompositeCommand.java:127) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl$SelfExecutionCommandService.execute(TaskCommandExecutorImpl.java:65) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.persistence.TaskTransactionInterceptor.execute(TaskTransactionInterceptor.java:54) [jbpm-human-task-jpa-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.persistence.jta.TransactionLockInterceptor.execute(TransactionLockInterceptor.java:67) [drools-persistence-jpa-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl.execute(TaskCommandExecutorImpl.java:40) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.impl.command.CommandBasedTaskService.complete(CommandBasedTaskService.java:156) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
......
Caused by: java.lang.NullPointerException
    at org.jbpm.services.task.wih.ExternalTaskEventListener.afterTaskCompletedEvent(ExternalTaskEventListener.java:116) [jbpm-human-task-workitems-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.events.TaskEventSupport.fireAfterTaskCompleted(TaskEventSupport.java:211) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.internals.lifecycle.MVELLifeCycleManager.taskOperation(MVELLifeCycleManager.java:359) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.impl.TaskInstanceServiceImpl.complete(TaskInstanceServiceImpl.java:153) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:76) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:39) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.CompositeCommand.execute(CompositeCommand.java:127) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl$SelfExecutionCommandService.execute(TaskCommandExecutorImpl.java:65) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.persistence.TaskTransactionInterceptor.execute(TaskTransactionInterceptor.java:54) [jbpm-human-task-jpa-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.drools.persistence.jta.TransactionLockInterceptor.execute(TransactionLockInterceptor.java:67) [drools-persistence-jpa-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl.execute(TaskCommandExecutorImpl.java:40) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]
    at org.jbpm.services.task.impl.command.CommandBasedTaskService.complete(CommandBasedTaskService.java:156) [jbpm-human-task-core-6.2.0.Final-redhat-4.jar:6.2.0.Final-redhat-4]