null expection

public void FilterFromSubject(string mailSubject)
{
    List<UnreadEmails> list = (List<UnreadEmails>)dgvUnreadMails.DataSource;
    List<UnreadEmails> filteredList = new List<UnreadEmails>();

    List<string> myUnreadMailList;
    try
    {
        filteredList = (List<UnreadEmails>)(from ci in list
                                            where ci.Subject.ToLower().Contains(mailSubject)
                                            select ci).ToList<UnreadEmails>();
        dgvUnreadMails.DataSource = filteredList;
        btnCancel.Enabled = true;
    }
    catch (System.Exception)
    {
        MessageBox.Show("Import E-mails first", "Empty Table", MessageBoxButtons.OK, MessageBoxIcon.Information);
        btnSave.Enabled = false;
    }
}