Hi!
Thats because this code is meant to be run from a terminal. If you are inside the Maya UI you already have an instance of QApplication, so you don't have to do it yourself. Doing it again will cause the two versions of QApplications to have conflicts and Maya to crash.
You should instead just instance the version of QApplication that already exists to bind it to your variable.
from PySide2.QtWidgets import QApplication, QMainWindow
import sys
app = QApplication.instance()
window = QMainWindow()
window.show()
app.exec_()
I hope it helps!
