Error Window not appearing in pyqt5 when login credentials are wrong

I am making a simple GUI in python using pyqt5. I am finally able to open the mainGUI window using QMainWindow by assigning the reference to self.win. Also, I want to be able to display the error message if the login is incorrect. However, the error window isn't showing up and exits with an exit code -1073740791 (0xC0000409)
Here is the snippet of my code.

main.py

import sys

from PyQt5.QtWidgets import QApplication, QMainWindow

import gui.loginGUI, gui.mainGUI, gui.errorGUI
import db.connect


class mainApp(QMainWindow):

    def __init__(self):
        self.app_Login = gui.loginGUI.login()
        self.app_Login.loginB.clicked.connect(
            lambda: self.loginCredentials(self.app_Login.username.text(), self.app_Login.pwd.text())
        )

    def loginCredentials(self, username, pwd):
        # check for login credentials
        if db.connect.connect(username, pwd).checkConn() != 0:
            self.app_Login.close()
            self.win = gui.mainGUI.Window(username, pwd)
        else:
            gui.errorGui.ErrorGUI("Wrong Credentials")
            self.app_Login.username.clear()
            self.app_Login.pwd.clear()


if __name__ == "__main__":
    try:
        app = QApplication(sys.argv)
        mainApp()
        app.exec_()
    except Exception as e:
        print(e)

errorGUI.py

from PyQt5.QtWidgets import QWidget, QMessageBox
from PyQt5.QtGui import QIcon

class Error(QWidget):

    def __init__(self, message):
        super().__init__()

        alert = QMessageBox()
        alert.setText(message)
        alert.setWindowTitle("Error")
        alert.exec_()