Note, that folder returned by SPList.RootFolder does not have corresponding SPListItem object.

So, SPListObject.RootFolder.Item will return null.

Note that you can use the rootfolder to get the internal name of the list, something like this extensionmethod:

public static SPList GetListByInternalName(this SPWeb web, string listInternalName)
{

        var list = (from SPList l in web.Lists
                        where l.RootFolder.Name.Equals(listInternalName, StringComparison.InvariantCulture)
                        select l).FirstOrDefault();
        if (list == null)
        {
            throw new FileNotFoundException(string.Format(
                    "SPList '{0}' not found on web '{1}'", listInternalName, web.Url));
        }
        return list;
}