The Exception is thrown as if the RenderMode is not set to Flat. If the GroupStyleSelector is defined before the RenderMode, the issue is not reproduced.
Currently the only option is to call the Expand/Collapse groups API, which takes noticable time to execute.
Currently, when RenderMode Flat is enabled, RadPropertyGridd uses a custom navigation behavior. The default framework navigation is available only when RenderMode is Hierarchical.
Three separate modes for RadPropertyGrid's ordering according to its states. OrderIndex should be respected in Sorted state/Grouped state/Both states (default value).
This issue has been reopen as the fix for it turned out to cause other problems.