I tried to reproduce the exception in Visual Studio 2013 but to no avail. The exception is thrown by RadTreeView collapse-expand animation but the latter is working as expected on our side.
I suggest that you went through every project in your solution that refers our binaries and delete the bin & obj folders (you have to close Visual Studio to be able to achieve this). Then open the IDE again and clean and rebuilt the solution. You should be able to see all controls without errors in Design View.
If you still reproduce the unwanted behaviour please send us a runnable version of your project so that we can reproduce and inspect it locally. Without any sample that reproduces this we are unable to help you.
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