QGridLayout in PyQt5

[DiGol]

I can't figure out why when I run the code the button(self.ui.btn) disappears. But without self.layout() it remains.

from PyQt5.QtWidgets import QWidget, QApplication, QGridLayout
from PyQt5 import uic
import sys

class App(QWidget):
    def __init__(self):
        super().__init__()
        self.design()
        self.layout()
    def design(self):
        self.ui = uic.loadUi('form.ui')
        self.ui.show()
    def layout(self):
        self.lay = QGridLayout()
        self.lay.setSpacing(10)
        self.lay.addWidget(self.ui.btn, 0, 1)
        self.setLayout(self.lay)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())

[Axel-Erfurt]

Whats the content of form.ui ?