关闭QFileDialog时报错，打开文件的方式不对吗？?报错

开发环境：

Python3.5+Qt Designer+PyQt5+PyChram

先用Qt Design 绘制一个Dialog窗口，添加了一个pushButton，保存ui，用pyuic5将ui转成ui_dialog.py,其代码如下：

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName("Dialog")
        Dialog.resize(400, 300)
        self.pushButton = QtWidgets.QPushButton(Dialog)
        self.pushButton.setGeometry(QtCore.QRect(160, 140, 75, 23))
        self.pushButton.setObjectName("pushButton")

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

    def retranslateUi(self, Dialog):
        _translate = QtCore.QCoreApplication.translate
        Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
        self.pushButton.setText(_translate("Dialog", "OK"))

把ui_dialog.py一起拷贝到qdialog project下，写main函数，代码如下：

from PyQt5.QtWidgets import QFileDialog, QDialog, QApplication, QMessageBox
from ui_dialog import *
import sys

class Dialog(QDialog, Ui_Dialog):
    def __init__(self, parent=None):
        super(Dialog, self).__init__(parent)
        self.setupUi(self)
        self.pushButton.clicked.connect(self.ok_btn)

    def ok_btn(self):
        fileName = QFileDialog.getOpenFileName(self)
        file = open(fileName)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    mainWindow = Dialog()
    mainWindow.show()
    sys.exit(app.exec_())

运行后弹出界面如下：

点击OK后弹出QFileDialog对话框，此时如果关闭右上角的小叉后就会报错。

感觉好像是main里面的

file = open(fileName)

出了问题。是打开的方式不对吗？