from PyQt5 import QtWidgets
from PyQt5.QtWidgets import QMessageBox

from decryptwindow import Ui_Dialog  # importing our generated file

import sys
import os

class mywindow(QtWidgets.QMainWindow):
    def __init__(self):
        super(mywindow, self).__init__()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self)


ini_file = 'config.ini'
ini_file_exists = os.path.isfile(ini_file)


if ini_file_exists:
    # just for testing
    print(ini_file + ' is found')
    # let`s show the main window
    app = QtWidgets.QApplication([])
    application = mywindow()
    application.show()
    sys.exit(app.exec())
else:
    # just for testing
    print(ini_file + ' is not found')
    app = QtWidgets.QApplication([])
    application = mywindow()
    application.show()
    error_dialog = QtWidgets.QErrorMessage()
    error_dialog.showMessage(ini_file + ' is not found')
    # after message I would like just close the application
    sys.exit(app.exec())