That's exactly when I receive this error, when I click on the arrow or type in something in combobox. I do not see any difference between yours and mine code. Can you please point me what changes should I do in order this to work?
I have tried to replicate the described issue, using 2013.2.611 version of our controls, but to no avail. Therefore, I am attaching the sample project I have tested with. You could give it a try at your end and test the behavior. Additionally, let us know how the implementation at your end differs, so we could replicate the problem locally. In addition, here is a video, demonstrating the behavior.
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