Not able to show menubar and title of the window in my PyQt5 application - how to fix?

Question

In my PyQt5 application I have made a subclass of QMainWindow to customize the main window. But with this design, I am not able to display the title of the main window and also not able to show the menu bar at the top.
If I uncomment the window.show() statement in my code, that pops-up 2 separate windows - and in one of them it can show and display the title and menu but my purpose is not met. How can I show the title, menu and the QStackedLayout that I have created in one single window ?

Below is my code :

import sys

from PyQt5.QtCore import QSize, QRect, Qt
from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton, QAction, QHBoxLayout, QVBoxLayout, QLabel, QWidget, QStackedLayout
from PyQt5.QtGui import QIcon


class Ui(QWidget):

    def setupUi(self, Main):
        #print (dir(Main))
        self.setWindowTitle("My App")
        Main.setObjectName("Main")
        Main.setFixedSize(900, 500)
        self.width = 900
        self.height = 500

        self.setFixedSize(self.width, self.height)
        
        menuBar = self.menuBar()
        fileMenu = menuBar.addMenu('&File')
       
        self.menu = QStackedLayout()

        self.mainMenu = QWidget()
        self.nextMenu = QWidget()

        self.mainMenuUi()
        self.nextMenuUi()

        self.menu.addWidget(self.mainMenu)
        self.menu.addWidget(self.nextMenu)

    def mainMenuUi(self):
        print ("Inside mainMenuUi")
        self.mainMenu.setFixedSize(self.width, self.height)
        self.mainMenuText = QLabel("welcome view", self.mainMenu)
        self.mainMenuText.setGeometry(QRect(30, 120, 480, 200))
        self.mainMenuText.setStyleSheet("font: 14pt Century Gothic")
        self.mainMenuText.setAlignment(Qt.AlignLeading|Qt.AlignLeft|Qt.AlignTop)
        self.btn2 = QPushButton("Next", self.mainMenu)

    def nextMenuUi(self):
        print ("Inside nextMenuUi")
        self.nextMenu_layout = QVBoxLayout()
        self.nextMenu.setFixedSize(self.width, self.height)
        self.btn1 = QPushButton("Back")
        self.nextMenu_layout.addWidget(self.btn1)
        self.nextMenu.setLayout(self.nextMenu_layout)


# Subclass QMainWindow to customize your application's main window
class MainWindow(QMainWindow, Ui):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("My App window title")

        self.setupUi(self)

        self.btn1.clicked.connect(self.menuWindow)
        self.btn2.clicked.connect(self.menuWindow2)

    def menuWindow(self):

        self.menu.setCurrentIndex(0)

    def menuWindow2(self):

        self.menu.setCurrentIndex(1)



app = QApplication(sys.argv)

window = MainWindow()
#window.show()

app.exec()

Answer 1

Qt Main Window Framework
...
Note: Creating a main window without a central widget is not supported. You must have a central widget even if it is just a placeholder.

---

What @musicamante wrote to you looks like this:

import sys
from PyQt5.QtCore import QRect, Qt
from PyQt5.QtWidgets import QApplication, QMainWindow, QPushButton, \
    QAction, QVBoxLayout, QLabel, QWidget, QStackedLayout, QStyle


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("My App window title")
        
        self.centralwidget = QWidget()
        self.setCentralWidget(self.centralwidget)
        
        menuBar = self.menuBar()
        fileMenu = menuBar.addMenu('&File')        
        openFile = QAction(self.style().standardIcon(QStyle.SP_MessageBoxCritical), 
            'Open', self)
        fileMenu.addAction(openFile)  

        self.width = 900
        self.height = 500
        self.resize(self.width, self.height)

        self.mainMenu = QWidget()
        self.nextMenu = QWidget()
        
        self.mainMenuUi()
        self.nextMenuUi()
        
        self.menu = QStackedLayout(self.centralwidget)
        self.menu.addWidget(self.mainMenu)
        self.menu.addWidget(self.nextMenu)

        self.btn1.clicked.connect(self.menuWindow)
        self.btn2.clicked.connect(self.menuWindow2)

    def mainMenuUi(self):
        self.mainMenuText = QLabel("welcome view", self.mainMenu)
        self.mainMenuText.setGeometry(QRect(30, 120, 480, 200))
        self.mainMenuText.setStyleSheet("font: 14pt Century Gothic")
        self.mainMenuText.setAlignment(Qt.AlignLeading|Qt.AlignLeft|Qt.AlignTop)
        self.btn2 = QPushButton("Next", self.mainMenu)

    def nextMenuUi(self):
        self.nextMenu_layout = QVBoxLayout()
        self.btn1 = QPushButton("Back")
        self.nextMenu_layout.addWidget(self.btn1)
        self.nextMenu.setLayout(self.nextMenu_layout)

    def menuWindow(self):
        self.menu.setCurrentIndex(0)

    def menuWindow2(self):
        self.menu.setCurrentIndex(1)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec())