Error delegate to signal

/**
 * Buku Telepon
 * Aplikasi untuk menyimpan buku telepon atau kontak
 *
 * @author    Agus Susilo
 * @date      13 November 2015 22:00
 * @version   0.1
 * @copyright (C) 2015 Susilolab
 */
namespace PhoneBook
{
    public class PhoneBookForm: Gtk.Application
    {
        public delegate void ToolButtonEvent();

        private Gtk.Builder builder;
        private Gtk.Window main_window;
        private Gtk.ImageMenuItem mnPhoneBook;
        private Gtk.ImageMenuItem mnQuit;
        private Gtk.ImageMenuItem mnAddGroup;
        private Gtk.ImageMenuItem mnAbout;
        private Gtk.ImageMenuItem mnRefresh;
        private Gtk.ScrolledWindow swPhoneBook;
        private Gtk.ToolItem tItemAdd;
        private Gtk.ToolItem tItemEdit;
        private Gtk.ToolItem tItemDelete;
        private Gtk.Toolbar toolbar;

        private Gtk.Box box;
        private Gtk.Box boxGrid;
        private Gtk.TreeView view;
        private Gtk.ListStore list_store = new Gtk.ListStore(6, typeof(string), typeof(string), typeof(string),
                typeof(string), typeof(string), typeof(string));
        private Sqlite.Database db;
        private const string dbName = "share/data/phonebook.db";

        public PhoneBookForm() {
            Object(application_id: "com.susilolabs.phonebook", flags: ApplicationFlags.FLAGS_NONE);
        }

        protected override void activate() {
            try {
                builder = new Gtk.Builder();
                builder.add_from_file("share/ui/phonebook.glade");
                builder.connect_signals(null);

                this.main_window = builder.get_object("winPhoneBook") as Gtk.Window;
                this.main_window.application = this;
                //this.main_window.set_position(WindowPosition.CENTER);

                swPhoneBook = builder.get_object("swPhoneBook") as Gtk.ScrolledWindow;
                box = builder.get_object("box1") as Gtk.Box;
                mnPhoneBook = builder.get_object("mnPhoneBook") as Gtk.ImageMenuItem;
                mnPhoneBook.activate.connect(mnPhoneBook_activate);

                mnRefresh = builder.get_object("mnRefresh") as Gtk.ImageMenuItem;
                mnRefresh.activate.connect(refresh_table_contact);

                mnQuit = builder.get_object("mnQuit") as Gtk.ImageMenuItem;
                mnQuit.activate.connect( () => {
                    this.quit();
                });

                mnAddGroup = builder.get_object("mnAddGroup") as Gtk.ImageMenuItem;
                mnAddGroup.activate.connect( () => {
                    AddGroup group = new AddGroup.with_parent_window(this.main_window);
                });

                mnAbout = builder.get_object("mnAbout") as Gtk.ImageMenuItem;

                add_toolbar();
                components_init();
                db_init();
                builder.connect_signals(this);
                this.main_window.show_all();

            }catch(Error e) {
                Utils.MessageBox(this.main_window, e.message, "Error saat memuat layout file");
                stderr.printf("%s\n", e.message);
            }
        }

        private Gtk.ToolItem new_item_button(string label, Gtk.Image img, ToolButtonEvent? evt=null) {
            var item = new Gtk.ToolItem();
            var btn = new Gtk.Button.with_label(label);
            btn.set_image(img);
            if(evt != null) {
                ToolButtonEvent ev = evt;
                btn.clicked.connect(ev());
            }
            btn.set_margin_left(5);
            btn.set_margin_top(5);
            btn.set_margin_right(5);
            btn.set_margin_bottom(5);

            item.add(btn);
            return item;
        }

        private void add_toolbar() {
            toolbar = builder.get_object("toolbar1") as Gtk.Toolbar;

            var imgAdd = new Gtk.Image.from_file("share/icons/add.png");
            ToolButtonEvent evtAdd = () => { stdout.printf("Tombol tambah diklik.\n"); };
            tItemAdd = new_item_button("Tambah", imgAdd, evtAdd);
            toolbar.add(tItemAdd);

            var imgEdit = new Gtk.Image.from_file("share/icons/pencil.png");
            tItemEdit = new_item_button("Ubah", imgEdit);
            toolbar.add(tItemEdit);

            var imgDelete = new Gtk.Image.from_file("share/icons/delete.png");
            tItemDelete = new_item_button("Hapus", imgDelete);
            toolbar.add(tItemDelete);
        }

        private void mnPhoneBook_activate() {
            AddContactForm app = new AddContactForm.with_parent_window(main_window);
            app.set_parent_app(this);
        }

        private List<Contact> get_contact() {
            List<Contact> contacts = new List<Contact>();

            File dbFile = File.new_for_path(dbName);
            int ec = 0;
            if(dbFile.query_exists(null)) {
                ec = Sqlite.Database.open(dbFile.get_path(), out db);
            }

            if(ec != Sqlite.OK) {
                stderr.printf("Tidak dapat membuka database: %s\n", db.errmsg());
            }

            string sql = "SELECT pb.id AS id, c.name AS cat_name, pb.name, pb.company, pb.title, ";
            sql += "pb.phone_number, pb.email FROM phone_book pb LEFT JOIN category c ";
            sql += "ON pb.category_id=c.id;";
            string errMsg;
            string[] res;
            int nrows;
            int ncols;

            ec = db.get_table(sql, out res, out nrows, out ncols, out errMsg);
            if(ec != Sqlite.OK) {
                stderr.printf("Error saat select: %s\n", errMsg);
            }

            int max = nrows * ncols + ncols;
            for(int i=0; i<max; i=i+ncols) {
                Contact  contact = new Contact();
                if(i == 0)
                    continue;
                else {
                    for(int j=0; j<ncols; j++) {
                        if(j == 0) {
                            contact.id = int.parse(res[i+j]);
                        }else if(j == 1) {
                            contact.category_name = res[i+j];
                        }else if(j == 2) {
                            contact.name = res[i+j];
                        }else if(j == 3) {
                            contact.company = res[i+j];
                        }else if(j == 4) {
                            contact.title = res[i+j];
                        }else if(j == 5) {
                            contact.phone_number = res[i+j];
                        }else if(j == 6) {
                            contact.email = res[i+j];
                        }
                    }
                }

                if(i > 0) contacts.append(contact);
            }
            return contacts;
        }

        public void refresh_table_contact() {
            list_store.clear();

            Gtk.TreeIter iter;
            List<Contact> contact = get_contact();
            foreach(Contact c in contact) {
                list_store.append(out iter);
                list_store.set(iter, 0, c.title, 1, c.name, 2, c.phone_number, 3, c.category_name,
                    4, c.email, 5, c.company);
            }
            view.set_model(list_store);
        }

        private void show_table_contact() {
            string[] cols = { "Judul", "Nama", "No. HP", "Grup", "Email", "Perusahaan" };
            boxGrid = new Gtk.Box (Gtk.Orientation.VERTICAL, 0);
            Gtk.TreeIter iter;
            List<Contact> contact = get_contact();
            foreach(Contact c in contact) {
                list_store.append(out iter);
                list_store.set(iter, 0, c.title, 1, c.name, 2, c.phone_number, 3, c.category_name,
                    4, c.email, 5, c.company);
            }

            view = new Gtk.TreeView.with_model(list_store);
            view.headers_clickable = true;
            view.show_expanders = true;
            boxGrid.pack_start(view, true, true, 0);
            swPhoneBook.add(boxGrid);

            Gtk.CellRendererText cell = new Gtk.CellRendererText();
            cell.editable = false;
            int i = 0;
            foreach(string col in cols) {
                view.insert_column_with_attributes(-1, col, cell, "text", i);
                i++;
            }
        }

        private void components_init() {
            show_table_contact();
        }

        private void db_init() {
            File dbFile = File.new_for_path("share/data/phonebook.db");
            int ec = 0;
            if(!dbFile.query_exists(null)) {
                ec = Sqlite.Database.open(dbFile.get_path(), out db);
            }

            if(ec != Sqlite.OK) {
                Utils.MessageBox(null, "Tidak dapat membuka database: " + db.errmsg(), "Error sqlite");
            }

            string sql = """
            CREATE TABLE IF NOT EXISTS phone_book(
                id INT PRIMARY KEY NOT NULL,
                category_id INT NOT NULL,
                name TEXT NOT NULL,
                company TEXT NOT NULL,
                title TEXT NOT NULL,
                phone_number TEXT NOT NULL,
                email TEXT NOT NULL
            );

            CREATE TABLE IF NOT EXISTS category(
                id INT PRIMARY KEY NOT NULL,
                name TEXT NOT NULL
            );
            """;

            string err;
            ec = db.exec(sql, null, out err);
            if(ec != Sqlite.OK) {
                Utils.MessageBox(null, "Error: " + err, "Error saat membuat tabel");
            }
        }

        public static int main(string[] args) {
            PhoneBookForm app = new PhoneBookForm();
            return app.run(args);
        }
    }
}