Thanks for your interest in Syncfusion products.
We have analyzed this reported scenario and we have created the simple sample as per your scenario. We are little bit unclear with your scenario. In that sample, the groups has been collapsed/expanded at run time using button-click event. Could you please provide with the below details,
· Please let us know how the 3rd PivotRow is added in your sample whether at runtime or not?
· Please provide us the stack trace of this exception and modify the attached sample with your scenario or provide us with the video of your sample.
These would be more helpful for us to provide the exact details at the earliest.