I cannot understand what is the issue that you encounter when you try to host that column selector in the ControlPanelItems section of the RadGridView? If you set only one item it is expected the control to visualize only one button in all the available space.
You can find SDK example
demonstrating how this panel can be used to host more than one item. Please take a look at it and let us know if you need any further assistance.
Pavel R. Pavlov
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