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.

从 new_debugger.py 中 quit 处的 traceback 以及 commands.py 中 debugger_cmd 处的命令分发开始。重现列出的 :Dbg 序列，然后跟踪调试器状态的创建和关闭过程；当 :Dbg quit 和 :Dbg . 无需重启 Vim 即可再次正常工作时，就算完成。

由索引模型根据 Issue 内容生成。