kivy: have tcp event open popup - kivy

Question

I'm new to kivy, so apologies if I've missed something really basic here but I am struggling to understand both the behavior I'm getting as well as my inability to get the outcome I'm after.
The goal
The kivy app should be listening for TCP events from an external server, with certain triggers causing various stuffs in the kivy app. In this case, I want the external server to be able to open a popup in the app.
Illustrative Code
I have adopted the following (working) demo code from another SO answer that I can no longer seem to find, so apologies for the plagiarism but it is not intentional.
server.py
import socket
serversocket = socket.socket()
host = 'localhost'
port = 54545
serversocket.bind(('', port))
serversocket.listen(1)
clientsocket,addr = serversocket.accept()
print("got a connection from %s" % str(addr))
while True:
msg = input("> ")
clientsocket.send(msg.encode('utf-8'))
client.py
import socket
class MySocket:
def __init__(self,host="localhost",port=54545):
self.sock = socket.socket()
self.sock.connect((host, port))
def get_data(self):
return self.sock.recv(1024)
main.py
import kivy
from kivy.app import App
from kivy.event import EventDispatcher
from kivy.uix.gridlayout import GridLayout
from kivy.uix.label import Label
from kivy.uix.popup import Popup
from threading import Thread
from client import *
kivy.require('1.9.1')
class MyEventDispatcher(EventDispatcher):
def __init__(self, **kwargs):
self.register_event_type('on_test')
super(MyEventDispatcher, self).__init__(**kwargs)
def do_something(self, value):
self.dispatch('on_test', value)
def on_test(self, *args):
print('I am dispatched', args)
CustomPopup().open()
class CustomPopup(Popup):
pass
class MainScreen(Label):
def __init__(self, **kwargs):
super(MainScreen, self).__init__(**kwargs)
self.sock = MySocket()
Thread(target=self.get_data).start()
def get_data(self):
while True:
self.text = self.sock.get_data().decode('utf-8').strip()
if self.text == 'click':
MyEventDispatcher().do_something(self.text)
class MyApp(App):
def build(self):
return MainScreen()
if __name__ == "__main__":
MyApp().run()
my.kv
<MainScreen>:
Button:
text: 'I do nothing'
<CustomPopup>:
title: 'Popup window'
size_hint: .5, .5
auto_dismiss: False
GridLayout:
cols: 1
Label:
size_hint: .9, .9
halign: 'center'
valign: 'middle'
text: 'message text goes here'
text_size: self.width, None
Button:
text: 'A: Close'
on_release: root.dismiss()
Button:
text: 'B: Close'
on_release: root.dismiss()
This all works in that I can send the trigger word click from the server and the popup will open, it has multiple placeholder buttons that work. Hooray.
The problem, however, is that I seem to require the dummy button on the main app screen, which I do not want. If I simply eliminate the <MainScreen> bit from the .kv file, sending the click keyword leads to:
I am dispatched ('click',)
Segmentation fault (core dumped)
but I can send any other string and it will display on screen as expected.
So, the basic questions are:
How do I get this to work as desired and
Why does placing a non-functional button on the screen make the popup work?
Thanks!