indicator application menu freezes irregularly

I have written a python program with an application indicator. Sometimes (about 1 over 3 times the complete program is started afresh), but not regularly, the indicator menu shows, the items are getting focus, but nothing happens when clicking.

I started the program from a terminal window, and expect that on clicking on any of the 4 buttons, output is printed to the terminal window, and at clicking on button 4 the program is also ended and the application icon removed. After most starts of the program, that happens, but sometimes (about 1 in 3) the buttons appear correctly, but nothing happens when they are clicked.

A simplified version of my program is:

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk as gtk
gi.require_version('AppIndicator3', '0.1')
from gi.repository import AppIndicator3 as appindicator

def maken_indicatorscherm():
   APPINDICATOR_ID = 'ID_App_test'
   indicator = appindicator.Indicator.new(APPINDICATOR_ID, "whatever", appindicator.IndicatorCategory.APPLICATION_STATUS)
   indicator.set_status(appindicator.IndicatorStatus.ACTIVE)
   indicator.set_menu(build_menu())
   gtk.main()

def build_menu():
   menu = gtk.Menu()
   item_open = gtk.MenuItem('Test 1')
   item_open.connect('activate', laat_EA_zien)
   menu.append(item_open)
   item_overzicht = gtk.MenuItem('text 2')
   item_overzicht.connect('activate', overzicht_EA)
   menu.append(item_overzicht)
   item_start = gtk.MenuItem('text 3')
   item_start.connect('activate', verberg_EA)
   menu.append(item_start)
   item_quit = gtk.MenuItem('Quit')
   item_quit.connect('activate', quitGTK)
   menu.append(item_quit)
   menu.show_all()
   return menu

def quitGTK(source):
   print "Quit program"
   gtk.main_quit()

def laat_EA_zien(source):
   print "show text 1"

def overzicht_EA(source):
   print "show text 2"

def verberg_EA(source):
   print "show text 3"

if __name__ == '__main__':
   maken_indicatorscherm()

I am using ubuntu:
Description: Ubuntu 16.04.2 LTS
Release: 16.04
with python:
2.7.12 (default, Nov 19 2016, 06:48:10)
[GCC 5.4.0 20160609]