Example in README does not work

[pkuwwt]

The example in dir examples works.

p = Pager()
p.add_source(GeneratorSource(generate_a_lot_of_contents())
p.run()

However, the example in README.md does not work:

p = Pager(GeneratorSource(generate_a_lot_of_contents())
p.run()

The error is

File "/usr/local/Cellar/python@3.9/3.9.0_2/Frameworks/Python.framework/Versions/3.9/lib/python3.9/weakref.py", line 405, in __getitem__
    return self.data[ref(key)]
KeyError: <weakref at 0x104c7e720; to 'DummySource' at 0x103fa6e80>

It is tested on MacOS python3.7, python3.9.