currentText() doesn't collect data from field

My program is currently divided into 2 files: one with GUI and second with code. In GUI, there's button I want to use to collect the data from text fields and put them into variables:

self.dodaj.clicked.connect(Dodaj_Ksiazke.pobierz_dane)

It's supposed to call the function "pobierz_dane". Inside the code there is class "Main" where above function is defined:

def pobierz_dane(self):
    print("Pobieranie danych z formularza do zmiennych")
    print(self.ui.isbn.currentText())
    isbn = self.ui.isbn.currentText()
    print(isbn)

As you can see I tried various ways to confirm it's working. When I start the program and click the button the first print gets displayed but not the 2nd or 3rd one, this is where program crashes with error:

Process finished with exit code -1073740791 (0xC0000409)

Text field I'm trying to get the text from is defined like this:

    self.isbn = QtWidgets.QLineEdit(self.centralwidget)
    self.isbn.setObjectName("isbn")
    self.gridLayout_2.addWidget(self.isbn, 1, 2, 1, 1)

This is the beginning of GUI deifnition:

class Ui_Dodaj_Ksiazke(object):
    def setupUi(self, Dodaj_Ksiazke):

And Main class from file with code:

class Main(QtWidgets.QMainWindow,Ui_Dodaj_Ksiazke):
    def __init__(self):
       QtWidgets.QMainWindow.__init__(self)
       self.ui= Ui_Dodaj_Ksiazke()
       self.ui.setupUi(self)

Since the first print is working I guess the button correctly calls the function. But I can't nail why it's not working for the text field as I don't get any errors when running the program.

Problem was fixed by switching from currentText() to displayText() as there is no attribute 'currentText' in QLineEdit.

isbn = self.ui.isbn.displayText()

Text is taken from text field stored correctly in variable, displayed by print() and program doesn't crash.