Cannot save Task

Flowable Engine
1

Hello everyone.
I would have to do a very simple thing: use TaskQuery to retrive a Task list. Then, for each task, update the task description:

List<Task> tasks = processEngine.getTaskService().createTaskQuery().active().list();
for (Task task : tasks) {
    task.setDescription("blablabla");
    processEngine.getTaskService().saveTask(task);
}

When I run it, I have the error:

CommandContext Error while closing command context
java.lang.NullPointerException
at org.flowable.engine…getOriginalAssignee(ActivityInstanceEntityManagerImpl.java:206)
at org.flowable.engine…recordActivityTaskInfoChange(ActivityInstanceEntityManagerImpl.java:187)

Where I’m wrong ?

I use Flowable 6.4.1

Thanks.
Gigi