Tab completion for dict key causes crash #917

my_dict = {
'Circle': {'Ellipse',
           'Shape',
           'ShapeDriver'},

'Ellipse': {'Shape'},

'Rectangle': {'Shape'},

'Shape': {'Circle'},

'ShapeDriver': {'Circle',
                'Ellipse',
                'Shape'},

'Square': {'Rectangle'}
}

~ % bpython
bpython version 0.21 on top of Python 3.8.2 /Library/Developer/CommandLineTools/usr/bin/python3
>>> my_dict = {
... 'a/Circle': {'Ellipse',
...            'Shape',
...            'ShapeDriver'},
... 
... 'a/Ellipse': {'Shape'},
... 
... 'a/Rectangle': {'Shape'},
... 
... 'a/Shape': {'Circle'},
... 
... 'a/ShapeDriver': {'Circle',
...                 'Ellipse',
...                 'Shape'},
... 
... 'a/Square': {'Rectangle'}
... }
>>> my_dict['b/banana']
Traceback (most recent call last):
  File "/Users/arian/Library/Python/3.8/bin/bpython", line 8, in <module>
    sys.exit(main())
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsies.py", line 201, in main
    exit_value = repl.mainloop(True, paste)
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsies.py", line 121, in mainloop
    self.process_event_and_paint(e)
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsies.py", line 85, in process_event_and_paint
    self.process_event(e)
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsiesfrontend/repl.py", line 615, in process_event
    return self.process_key_event(e)
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsiesfrontend/repl.py", line 757, in process_key_event
    self.on_tab()
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/curtsiesfrontend/repl.py", line 891, in on_tab
    cursor_and_line = self.matches_iter.substitute_cseq()
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/repl.py", line 297, in substitute_cseq
    self.update(
  File "/Users/arian/Library/Python/3.8/lib/python/site-packages/bpython/repl.py", line 318, in update
    self.start, self.end, self.current_word = self.completer.locate(
TypeError: cannot unpack non-iterable NoneType object