Free move in call activity mutipart instance case

Hello god,
I have process definition like below:
start—>parentTask1—>call activity(mutipart instance sub process) —>parentTask2—end
and in my sub process the definition like:
and for the call activity node,it is mutipart with the config like below:
<bpmn2:callActivity id=“subProcess” calledElement=“subProcess” flowable:inheritVariables=“true”>
<bpmn2:multiInstanceLoopCharacteristics flowable:collection=“assigneeList” flowable:elementVariable=“assignee”>

now,i would like to move from parentTask1 to subTask2 and i use the api:
it works,but it seems that there are some missing in the execution table and when i complete the subTask2, i got an NullPoint execption
do you hace any solution about my case? thanks