QMessageDialog does not run

import sys

from PyQt5.QtCore import QLockFile
from PyQt5.QtWidgets import QApplication
from PyQt5.QtWidgets import QMessageBox

from window import MainWindow


if __name__ == "__main__":
    lock_file = QLockFile("app.lock")

    try:
        if lock_file.tryLock():
            app_object = QApplication(sys.argv)

            window = MainWindow()
            window.show()

            app_object.exec()
        else:
            error_message = QMessageBox()
            error_message.setIcon(QMessageBox.Warning)
            error_message.setWindowTitle("Error")
            error_message.setText("Chess Fighter is already running!")
            error_message.setStandardButtons(QMessageBox.Ok)
            error_message.exec()
    finally:
        lock_file.unlock()

### The error I get is:    "QWidget: Must construct a QApplication before a QWidget"  ###