Как оставить окно открытым в Python PyQt

Алек ответил на вопрос, но если вам все еще непонятно, вот исправленный код.

import sys
from PyQt5.QtWidgets import QMainWindow, QLabel, QPushButton, QMessageBox, QApplication
from PyQt5.QtCore import pyqtSignal, pyqtSlot
from PyQt5.QtGui import QIcon, QFont
class MainWindow(QMainWindow):
    switch_window=pyqtSignal(str)
    def __init__(self):
        super().__init__()
        self.initUI()

    def initUI(self):
        #Initialize
        self.setGeometry(1000, 300, 1200, 800)
        self.setWindowTitle('Sensorlyze')
        self.setWindowIcon(QIcon('biosensor.jpg'))
        icon = QIcon('biosensor.jpg')

        # Add Text
        l1= QLabel("Welcome to SensorLyze",self)
        l1.move(25, 350)
        # l1.setWordWrap(True)
        l1.setFont(QFont('Calibri',15))
        l1.adjustSize()
        l2 = QLabel("A software to simply sensor analytics", self)
        l2.move(25, 400)
        l2.setFont(QFont('Calibri', 10))
        l2.adjustSize()

        #Add Buttons
        button1 = QPushButton('Start',self)
        button1.resize(button1.sizeHint())
        button1.clicked.connect(self.start_clicked)
        button1.move(60, 450)
        button2 = QPushButton('Exit', self)
        button2.resize(button2.sizeHint())
        button2.clicked.connect(self.exit_clicked)
        button2.move(240, 450)

    def start_clicked(self):
        self.window = QMainWindow()
        self.window.setGeometry(300, 500, 500, 500)
        self.window.setWindowTitle('Hello')
        self.window.show()
        # win.hide()

    def exit_clicked(self):
        msgBox = QMessageBox()
        msgBox.setIcon(QMessageBox.Information)
        msgBox.setText("Are you sure you want to exit?")
        msgBox.setWindowTitle("Exit Sensorlyze")
        msgBox.setStandardButtons(QMessageBox.Ok | QMessageBox.Cancel)
        msgBox.buttonClicked.connect(self.msgButtonClick)
        returnValue = msgBox.exec()

        if returnValue == QMessageBox.Ok:
            exit()

    def msgButtonClick(self, i):
        print("Buttonclickedis:", i.text())


stylesheet = """
    QMainWindow {
    background-image: url("C:/Users/admin/Desktop/Sensorlyze/biosensor.jpg"); 
    background-repeat: no-repeat; 
    background-position: center;
    }
"""

# def switch(self):
#         self.switch_window.emit(self.line_edit.text())

def main():
    app = QApplication(sys.argv)
    app.setStyleSheet(stylesheet)     # <---
    win=MainWindow()
    win.show()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()