Case_id attribute of a <case> wasn't set

Hi forum,

I would like to ask this error. that is happing when I finish the form by click complete. but actually it not save the data and show error.

The case_id attribute of a wasn't set. Source: n0:case tag in namespace:

I thinks that it related to parent case saving? or what?
but my problem is while I update the group activity. by update the service providers. in beginning we registered group activity but we skip the service providers and then we want to add service provider into that group activity.

So after we adding service providers and hit complete the form it not finish and show this error?

please help me the way to fix it.

Thanks regard,