AttributeError: Debugger instance has no attribute 'bend'

After some fooling around with

:Dbg b
(timeout)
:Dbg quit
:Dbg -
(timeout)
:Dbg quit
:Dbg .

I got an error message that I dismissed by accident without being able to read. Now I cannot :Dbg quit -- I get this traceback:

```
Traceback (most recent call last):
File "", line 1, in
File "/home/mg/.vim/plugin/vim_debug/commands.py", line 44, in debugger_cmd
cmd['function'](*args)
File "/home/mg/.vim/plugin/vim_debug/new_debugger.py", line 187, in quit
self.bend.close()
AttributeError: Debugger instance has no attribute 'bend'
```

":Dbg ." now doesn't work either -- prints the usage message -- so there's no obvious way to debug anything without restarting vim.

Start with the traceback in new_debugger.py at quit and the command dispatch in commands.py at debugger_cmd. Reproduce the listed :Dbg sequence, then trace how the debugger state is created and closed; done means :Dbg quit and :Dbg . work again without restarting Vim.

Written by the indexing model from the issue text.