When the LevelText of the list style is empty, PdfFormatProvider throws an IndexOutOfRangeException while trying to draw the bullet.

Workaround: Ensure that the LevelText of the ListItemStyle is not an empty string:

foreach (ListStyle listStyle in this.radRichTextBox.Document.ListManager.GetAllListStyles())
{
    foreach (ListLevelStyle level in listStyle.Levels)
    {
        if (string.IsNullOrEmpty(level.LevelText))
        {
            level.LevelText = " ";
        }
    }
}