To reproduce:
1 Please change the font via selection list first.
2 Set text cursor to font input field and select the whole text (if not already done).
3 Press backspace or delete key.
4 An error message appears.

Workaround:
protected override void DropDownListFont_SelectedIndexChanged(object sender, PositionChangedEventArgs e)
{
    var ddl = sender as RadDropDownListElement;
    if (ddl.SelectedItem != null)
    {
        base.DropDownListFont_SelectedIndexChanged(sender, e);
    }
}