In one of our previous releases we changed the internal element structure of RadRibbonBar. This caused issues in old projects where different properties of RadRibbonBar are serialized at design time.
To overcome the exception you experience, please make sure that you remove all the serialized code which accesses the ribbon bar children from your designer file and introduce the desired settings either in code or redo them in design time. The error is raised because we have changed the element structure of RadRibbonBar, and the old code is trying to access elements, which are no longer there.
If you cannot manage to overcome the exception, please send us your project. We will investigate and address the issue. Thank you in advance.
the Telerik team