KIVY popup - how do I dismiss it from a compound content - popup

Question

I am trying to create a Kivy popup for general use
From my code below you may see that I am a beginner
I am trying to understand the KIVY language, ids, kivy properties, how to apply rules and the interaction between the KV and Python files
I will appreciate your answers and comments, that may teach me
Ho do I dismiss the popup?
Thank you for your aid
===================================================================
My KV file
'''
<MB_Body>
id: id_body
size_hint: 0.9, 0.9
Label:
id: id_label
size_hint: (0.6, 0.2)
pos_hint: {'x': 0.2, 'top': 0.9}
text: 'Sibs text'
Button:
text: 'Dismiss'
size_hint: 0.8, 0.2
pos_hint: {'x': 0.1, 'y': 0.1}
on_release: root.dismiss_msgbox()
id: pop
auto_dismiss: False
size_hint: 0.6, 0.4
title: 'si8bs title'
canvas.before:
Color:
rgba: 0, 1, 0, 1
Rectangle:
size: self.size
pos: self.pos
MB_Body:
'''
##########################################################
My PY file:
'''
from kivy.app import App
from kivy.uix.popup import Popup
from kivy.uix.floatlayout import FloatLayout
from kivy.properties import StringProperty, ObjectProperty
class MsgBox(Popup):
pass
class MB_Body(FloatLayout):
def dismiss_msgbox(self):
print('In Finis')
print(App.get_running_app())
dismiss() #---> This line is my problem. How do I dismiss the popup???????????
def showMsg(_title, _msg):
mb = MsgBox()
mb.title = _title
mb.content.ids.id_label.text = _msg
mypopup = mb
print(_msg)
mb.open()
'''