To reproduce:
class RadDataEntryCustomForm : RadDataEntry
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadDataEntry).FullName;
        }

    }
}

Workaround:
class RadDataEntryCustomForm : RadDataEntry
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadScrollablePanel).FullName;
        }

    }
}