Hi Christ,
Thanks for evaluating Syncfusion products.
We have tested the GroupBar control in UserControl.We are not able to get any exception when we try to add the GroupBarItem. We have attached the Simple sample here which illustrates that. If you feel still the issue persists could you please modify the sample and send it to us. It would be helpful to find an accurate solution.
Please let us know if you have any queries.
Regards,
David C
GroupBarSample_8c478d3.zip