void btnOK_Click(object sender, EventArgs e)
        {
            if (txtShortName.Text == string.Empty || txtLongName.Text == string.Empty)
            {
                Tools.UserInfoHandler.ShowError("Please indicate a short name and a long name for the add-on.");
                return;
            }

            if (CountryAdministrator.DoesCountryExist(txtShortName.Text))
            {
                Tools.UserInfoHandler.ShowError("Add-on named '" + txtShortName.Text + "' already exists.");
                return;
            }

            if (txtSymbol.Text != string.Empty && !CountryAdministrator.IsValidFlagFilePath(txtSymbol.Text))
            {
                return;
            }

            if (lstAddOnSystems.CheckedItems.Count == 0 || lstBaseSystems.CheckedItems.Count == 0)
            {
                Tools.UserInfoHandler.ShowError("Please select an add-on system and a base system.");
                return;
            }
            _addOnSystemName = lstAddOnSystems.SelectedItem.ToString();
            _baseSystemName  = lstBaseSystems.SelectedItem.ToString();

            if (_addOnSystemName == _baseSystemName)
            {
                Tools.UserInfoHandler.ShowError("Add-on system must be different from base system.");
                return;
            }

            DialogResult = DialogResult.OK;
            Close();
        }