Recalling a MainWindow from an other script on PyQGIS console

Question

I'm writing some scripts on Python console in QGIS. I have been stuck since a lot of time because I'm not able to recall a MainWindow class object from an other script. I've created a main menu with inside a combobox. The idea is that when the user selects an option from this dropdown list, an other window opens. The problem is that this window opens for just a moment and then closes itself. Here the code.

this is the main menu's script (proofRequest.py) (not whole)

import sys 
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
import proofRequest as pr

def startButton(self):
       
        
        x = self.comboBox.currentIndex()
        
        if x == 3:
           win = pr.MainWindow()
           win.show()

While this is the script of the window which I would open (always not whole)

import sys
from PyQt5.QtWidgets import *
from qgis.utils import iface 
from PyQt5.QtCore import *
from PyQt5.QtGui import *


class MainWindow(QMainWindow):
    
    def __init__(self, parent=None, *args):
        
        
        super(MainWindow, self).__init__(parent = parent)
        self.setWindowTitle("QDialog Example")
        self.resize(350, 300)
        #layout = QVBoxLayout()
        print("sono qui")
        self.button1Ok = QPushButton("Ok", self)
        self.button2Annulla = QPushButton("Annulla", self)
        self.button1Ok.setGeometry(30,180,130,40)
        self.button2Annulla.setGeometry(180,180,130,40)
        label = QLabel("Seleziona un comune con edifici, poi clicca su ok", self)
        label.setAlignment(Qt.AlignHCenter)
        label.setGeometry(10, 50, 330, 30)
        self.button1Ok.clicked.connect(self.avvia)
        self.button2Annulla.clicked.connect(self.cancella)

I tried also to use:

qgs = QgsApplication([], True)
qgs.initQgis()
exitcode = qgs.exec()
QgsApplication.exitQgis()
sys.exit(exitcode)

But QGIS crashes...