为什么点击按钮后直接关闭窗口了，没有弹出对话框呢？

为什么点击按钮后直接关闭窗口了，没有弹出对话框呢？

 # -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'frmMain.ui'
#
# Created by: PyQt5 UI code generator 5.5.1
#
# WARNING! All changes made in this file will be lost!
import sys
from PyQt5.QtWidgets import QApplication , QMainWindow
from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_frmMain(object):
    def setupUi(self, frmMain):
        frmMain.setObjectName("frmMain")
        frmMain.resize(415, 265)
        self.centralwidget = QtWidgets.QWidget(frmMain)
        self.centralwidget.setObjectName("centralwidget")
        self.btnFirst = QtWidgets.QPushButton(self.centralwidget)
        self.btnFirst.setGeometry(QtCore.QRect(150, 70, 75, 23))
        self.btnFirst.setObjectName("btnFirst")
        frmMain.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(frmMain)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 415, 23))
        self.menubar.setObjectName("menubar")
        frmMain.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(frmMain)
        self.statusbar.setObjectName("statusbar")
        frmMain.setStatusBar(self.statusbar)

        self.retranslateUi(frmMain)
        self.btnFirst.clicked.connect(self.btnFirstClike)
        QtCore.QMetaObject.connectSlotsByName(frmMain)


    def retranslateUi(self, frmMain):
        _translate = QtCore.QCoreApplication.translate
        frmMain.setWindowTitle(_translate("frmMain", "我的第一个窗口"))
        self.btnFirst.setText(_translate("frmMain", "第一个按钮"))

    def btnFirstClike(self):
        QtWidgets.QMessageBox.about(self,"测试","点击弹出窗口成功")

if __name__ == '__main__':
    '''    主函数    '''
    app = QApplication(sys.argv)
    mainWindow = QMainWindow()
    ui = Ui_frmMain()
    ui.setupUi(mainWindow)
    mainWindow.show()
    sys.exit(app.exec_())

    mainWindow.show()
    sys.exit(app.exec_())

第一句就是显示窗口，只不过第二句把进程关闭了，当然什么也不显示

那不按按钮的时候为什么不关闭呢？