Workaround: 
public class CustomRadRichTextEditor : RadRichTextEditor
{
    /// <summary>
    /// Gets or sets the width of the caret.
    /// </summary>
    [Browsable(false)]
    [Category(RadDesignCategory.AppearanceCategory)]
    public new float CaretWidth
    {
        get
        {
            float caretWidth = base.CaretWidth;
            if (float.IsNaN(caretWidth))
            {
                return 2;
            }

            return caretWidth;
        }
        set 
        { 
            base.CaretWidth = value; 
        }
    }
}