problem in QFileDialog with file explore when i close it without select any file

I use the QfileDialog to upload a video file from my device and it works fine the problem occurs when I close the file explore without select anything, then it closes everything(main interface) and shows this error.

Process finished with exit code -1073740791 (0xC0000409)

here is a code :the QfileDialog in the upload function :

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtGui import QPixmap
from PyQt5.QtWidgets import QFileDialog, QMessageBox
from moviepy.editor import VideoFileClip
import os
global VideoPath
VideoPath=None

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(671, 389)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(100, 50, 93, 28))
        self.pushButton.setObjectName("pushButton")
        # to call function
        self.pushButton.clicked.connect(self.check)
        self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_2.setGeometry(QtCore.QRect(100, 140, 93, 28))
        self.pushButton_2.setObjectName("pushButton_2")
        self.pushButton_2.clicked.connect(self.preview)

        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.actiondjwk = QtWidgets.QAction(MainWindow)
        self.actiondjwk.setObjectName("actiondjwk")

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

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "Upload"))
        self.pushButton_2.setText(_translate("MainWindow", "Preview"))
        self.actiondjwk.setText(_translate("MainWindow", "djwk"))

    #     upload function
    def upload(self):
        fname = QFileDialog.getOpenFileName(self, 'Open file',
                                            'c:\\', "Video Files (*.mp4)")
        global VideoPath
        VideoPath = fname[0]
        print(VideoPath)
        global video
        video = VideoFileClip(VideoPath)

    def check(self):
        try:
            self.upload()
            if round(video.duration) > 6:
                raise durationError
        except durationError:
            global VideoPath
            VideoPath = None

            ##set VideoPath to none so next steps will not work if the there is no video path

            msg = QMessageBox()
            msg.setIcon(QMessageBox.Critical)
            msg.setText("The duration of the video should not exceed 5 seconds ")
            msg.setWindowTitle("Error")
            msg.exec_()
            # print(VideoPath)
        # print(VideoPath)


import sys
from PyQt5 import QtWidgets, uic

from test import  Ui_MainWindow


class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self, *args, obj=None, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)
        self.setupUi(self)


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

# ****************************
class durationError(Exception):
    ...