How to check if user already selected an item inside QListWidget?

Question

I want to be able to differentiate whether a user selected an item of my QListWidget, because he may also have entered a free text instead. I tried by checking the currentIndex of the QListWidget, but it seems like the currentIndex function does not return None as I have been hoping. Is there an easy way or may I need to implement a onSelectionChanged method to know whether user selected an item? If I have to go the "method implementation" way, how could the user "unselect" his selection?

    def ok_clicked(self):
    self.sig_1_index = self.dialog_layout.data_series_sig_list_1.currentIndex()
    self.sig_2_index = self.dialog_layout.data_series_sig_list_2.currentIndex()
    self.text_2 = self.dialog_layout.data_series_operator.currentText()
    if self.sig_1_index != None: #doesnt work
        if self.sig_2_index != -1: #also doesnt work
            self.text_1 = self.dialog_layout.data_series_sig_list_1.itemFromIndex(self.sig_1_index).text()
            self.text_3 = self.dialog_layout.data_series_sig_list_2.itemFromIndex(self.sig_2_index).text()
        else:
            self.text_1 = self.dialog_layout.data_series_filter_1.toPlainText()
            self.text_3 = self.dialog_layout.data_series_sig_list_2.itemFromIndex(self.sig_2_index).text()
    else:
        if self.sig_2_index != None:
            self.text_1 = self.dialog_layout.data_series_sig_list_1.itemFromIndex(self.sig_1_index).text()
            self.text_3 = self.dialog_layout.data_series_filter_2.toPlainText()
        else:
            self.text_1 = self.dialog_layout.data_series_filter_1.toPlainText()
            self.text_3 = self.dialog_layout.data_series_filter_2.toPlainText()
    print(self.text_1 + self.text_2 + self.text_3)