Bug summary
When using "set_window_title" I get this error:
self.manager.set_window_title(title)
AttributeError: 'NoneType' object has no attribute 'set_window_title'

Located in this file and error:
File "..\lib\site-packages\matplotlib\backend_bases.py", line 2245, in set_window_title
self.manager.set_window_title(title)
AttributeError: 'NoneType' object has no attribute 'set_window_title'

Code for reproduction
self.canvas.set_window_title('Some Text')

• Operating system: Win 10
• Matplotlib version: 3.3.0rc1
• Matplotlib backend (print(matplotlib.get_backend())): matplotlib.backends.backend_qt5agg
• Python version: 3.7.7
• Other libraries: PyQt5: 5.15.0