To reproduce:
- Create a custom cell class and try to add an RadMultiColumnComboBoxElement to its children.

Workaround:
public class MyMCCBElement : RadMultiColumnComboBoxElement
{
    protected override void OnParentChanged(Telerik.WinControls.RadElement previousParent)
    {
        if (this.Parent.ElementTree != null)
        {
            base.OnParentChanged(previousParent);
        }
    }
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadMultiColumnComboBoxElement);
        }
    }
}