What does "Process finished with exit code 1" mean?

I tried to develop a simple currency program but I have a problem. When I click on Çevir, the program should calculate money (like an exchange). But I can’t do it. PyCharm writes Process finished with exit code 1 when I click Çevir

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtGui import QIcon
from PyQt5.QtWidgets import qApp


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
      ....(qtdesigner codes . i skip this part)


        self.pushButton.clicked.connect(self.cevirici)
        self.pushButton_2.clicked.connect(self.cikis)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)
   
    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label_2.setText(_translate("MainWindow", "Birinci Döviz"))
        self.label.setText(_translate("MainWindow", "İkinci Döviz"))
        self.label_3.setText(_translate("MainWindow", "Miktar"))
        self.label_4.setText(_translate("MainWindow", "Sonuç :"))
        self.pushButton.setText(_translate("MainWindow", "Çevir"))
        self.pushButton_2.setText(_translate("MainWindow", "Çıkış Yap"))
    
    def cevirici(self):
        import requests

        import sys

        url = "http://api.fixer.io/latest?base="

        birinci_doviz = self.comboBox.currentText()
        ikinci_doviz = self.comboBox_2.currentText()

        miktar = int(self.lineEdit.currentText())

        response = requests.get(url + birinci_doviz)

        json_verisi = response.json()


        self.lineEdit_2.setText(json_verisi["rates"][ikinci_doviz] * miktar)
    def cikis(self):
        qApp.quit()    

if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())