The Docking control doesn't support inheriting RadSplitContainers, RadPaneGroups and RadPanes yet. What I mean is it cannot load the layout, because it doesn't know how to create instances of your classes. You should also notice that inheriting RadSplitContainers and RadPaneGroups doesn't make much sense, because they are created and destroyed automatically. If you need to inherit controls, you should put SerializationTag to their instances. This will force the Docking control to reuse the old instances and it won't crash when loading the layout.
We are planning to include this new feature (allowing inheritance of the RadDocking
classes), but I cannot give you any concrete time frame for that.
the Telerik team