setParent(None) fail

from PyQt5.Qt import *
app =  QApplication([])

main1 = QMainWindow(None)
main1.show()

btn = QPushButton('delete parent', main1)
btn.clicked.connect(lambda: label.setParent(None))
btn2 = QPushButton('call show()', main1)
btn2.clicked.connect(lambda: label.show())

vlay = QVBoxLayout(main1)
vlay.addWidget(btn)
vlay.addWidget(btn2)
central = QWidget(main1)
central.setLayout(vlay)
main1.setCentralWidget(central)

main2 = QMainWindow(main1)
label = QLabel('test', main2)
main2.show()

app.exec()