I have found a solution to a bug when using the script on:
https://matplotlib.org/gallery/user_interfaces/embedding_in_qt_sgskip.html
with the Spyder IDE (v3.3.2)

If the script is run as presented, two issues are present:
Issue 1: The qapp.exec_() does not terminate when the window is closed.

This was resolved for me by adding:
QtWidgets.QApplication.setQuitOnLastWindowClosed(True)
before qapp.exec_() is called:

if not QtWidgets.QApplication.instance():
    qapp = QtWidgets.QApplication(sys.argv)
else:
    qapp = QtWidgets.QApplication.instance()
QtWidgets.QApplication.setQuitOnLastWindowClosed(True)

app = ApplicationWindow()
app.show()
qapp.exec_()

Issue 2: When this script is called multiple times in the IPython console, the Kernel dies and must be restarted. Per the comment thread on issue #4349, this is solved by replacing:

qapp = QtWidgets.QApplication(sys.argv)

with

if not QtWidgets.QApplication.instance():
    qapp = QtWidgets.QApplication(sys.argv)
else:
    qapp = QtWidgets.QApplication.instance()

Can this be added to the example script?

Matplotlib version

• Operating system: Windows 10
• Matplotlib version: 3.0.2
• Python version: 3.7