Qt dialog destructor bug

from PyQt5.QtWidgets import QApplication, QMainWindow, QDialog, QPushButton, QLineEdit, QFormLayout

class MainWnd(QMainWindow):

    def __init__(self):
        super().__init__()

        self.button = QPushButton('Open dialog')
        self.button.clicked.connect(self.show_dialog)
        self.setCentralWidget(self.button)

        self.dlg_widget = QLineEdit('Close dialog and try to open again')

    def show_dialog(self):
        dlg = Dlg(self)
        dlg.exec_()

class Dlg(QDialog):

    def __init__(self, parent=None):
        super().__init__(parent)

        self.layout = QFormLayout()
        self.layout.addWidget(self.parentWidget().dlg_widget)
        self.setLayout(self.layout)


app = QApplication([])
mwnd = MainWnd()
mwnd.show()
exit(app.exec_())