Problem with joinChild on datasource


on form ReqTransPo I added datasource InventTable. Now when I open form and select some rows and click on Function button this error message show:

Error executing code: ReqTransFormPO (object), method editReqTransMark called with invalid parameters.
(C)\Classes\ReqTransFormPO\markMultiMarked - line 8
(C)\Classes\ReqTransFormPO\buttonFunctionsClickedPre - line 4
(C)\Classes\FormMenuButtonControl\Clicked - line 8

The problem is in the class ReqTransPo in method markMultiMarked:

reqPoMark = reqPo_DS.getFirst(1,reqPo_DS.defaultMark());
while (reqPoMark.RecId)
reqPoMark = reqPo_DS.getNext();

if (_markCurrent)

Function reqPoMark.joinChild() returns InventTable instead of ReqTrans.

Please can someone help me?

Nobody know?