Python pyqt5 screen crash

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Python

GUI

PyQt

I have designed two screens in PyQt5 It is runnig fine on startup every button is working. But when I click back button on second screen that is Ui_Dialog to go to first screen then on first screen If I click again any button It gives error that Ui_Dialog is not defined which is previously working fine kindly check my back Button code I have attached both screens code snippet.

Screen 1 MainWindow

from PyQt5 import QtCore, QtGui, QtWidgets
from Login import *


class Ui_MainWindow(object):
    def adminLogin(self):
        try:
            self.w = QtWidgets.QDialog()  
            self.ui = Ui_Dialog()  
            self.ui.setupUi(self.w) 
            Ui_Dialog.CurrentUser = "admin"  
            self.w.show()  
        except Exception as e:
            print(e)

    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(572, 468)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        .
        .
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.btnDoctor.clicked.connect(self.adminLogin)
        self.btnDoctor.clicked.connect(MainWindow.close)

        self.btnPatient.clicked.connect(self.patLogin)
        self.btnPatient.clicked.connect(MainWindow.hide)

        self.btnExit.clicked.connect(QtWidgets.qApp.quit)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.btnDoctor.setText(_translate("MainWindow", "Doctor"))
        self.btnPatient.setText(_translate("MainWindow", "Patient"))
        self.btnExit.setText(_translate("MainWindow", "Exit"))
        self.nameLabel.setText(_translate("MainWindow", "Hospital Management System"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    app.exec_()

screen 2 Ui_Dialog

from PyQt5 import QtCore, QtGui, QtWidgets
import pymysql
from mainWindow import *
from AdminLogin import *
from Register import *
from PatientLogin import *

class Ui_Dialog(object):
    CurrentUser = ""

    def messageBox(self,title,message):
        msg=QtWidgets.QMessageBox()
        msg.setWindowTitle(title)
        msg.setText(message)
        msg.setStandardButtons(QtWidgets.QMessageBox.Ok)
        msg.exec_()

    def loginUser(self):
        try:
            username = self.user_txt.toPlainText()
            password = self.pass_txt.toPlainText()

            if username!="" and password!="":
                conn = pymysql.connect(host="localhost", user="root", password="", db="Hospital_Management_System")
                cur = conn.cursor()
                if self.CurrentUser=="admin":
                    query = "select * from admin where name=%s and password=%s"
                    data = cur.execute(query, (username, password))
                    if len(cur.fetchall()) > 0:
                        self.messageBox("Alert", "Welcome " + self.user_txt.toPlainText())
                        self.w = QtWidgets.QDialog()  # type of window you want to open
                        self.ui = Ui_adminLogin()  # object of window you want to open
                        self.ui.setupUi(self.w)  # initialize the object of window you want to open
                        self.w.show()  # show the required window
                        Ui_adminLogin.CurrentUser=username

                    else:
                        self.messageBox("Alert", "Wrong Credentials")

            else:
              self.messageBox("Alert","Please Fill all Credentials")

        except Exception as e:
            print(e)
    def back(self):
        try:
            self.w = QtWidgets.QMainWindow()  # type of window you want to open
            self.ui = Ui_MainWindow() # object of window you want to open
            self.ui.setupUi(self.w)  # initialize the object of window you want to open
            self.w.show()  # show the required window
        except Exception as e:
            print(e)

    def reg(self):
        if self.CurrentUser=="patient":
            self.w=QtWidgets.QDialog()
            self.ui=Ui_Register()
            self.ui.setupUi(self.w)
            self.w.show()
        
    def setupUi(self, Dialog):
        Dialog.setObjectName("MainWindow")
        Dialog.resize(513, 299)
        self.Username = QtWidgets.QLabel(Dialog)
        self.Username.setGeometry(QtCore.QRect(150, 70, 61, 21))
        self.Username.setObjectName("Username")
        self.Password = QtWidgets.QLabel(Dialog)
        self.Password.setGeometry(QtCore.QRect(150, 120, 47, 13))
        self.Password.setObjectName("Password")
        .
        .
        self.btnBack.setGeometry(QtCore.QRect(430, 10, 75, 23))
        self.btnBack.setObjectName("btnBack")
        self.loginButton.clicked.connect(self.loginUser)
        self.loginButton.clicked.connect(Dialog.hide)

        self.registerButoon.clicked.connect(self.reg)
        self.registerButoon.clicked.connect(Dialog.hide)

        self.btnBack.clicked.connect(self.back)
        self.btnBack.clicked.connect(Dialog.hide)

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.Username.setText(_translate("Dialog", "Username "))
        self.Password.setText(_translate("Dialog", "Password"))
        self.Forgot_Password.setText(_translate("Dialog", "Forgot Password?"))
        self.loginButton.setText(_translate("Dialog", "Login"))
        self.registerButoon.setText(_translate("Dialog", "Register"))
        self.btnBack.setText(_translate("Dialog", "Back"))


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Dialog = QtWidgets.QDialog()
    ui = Ui_Dialog()
    ui.setupUi(Dialog)
    Dialog.show()
    sys.exit(app.exec_())

What I have tried:

Every back button code available has same code like this