Error: Error executing lua callback: ...e/nvim/lazy/pommodoro-clock.nvim/lua/pommodoro-clock.lua:233: attempt to index field 'popup' (a nil value)

These are the lines that caused the following error:

https://github.com/jackMort/pommodoro-clock.nvim/blob/eb7a59109c82917ba1c386f730ecdb9ef778b8a5/lua/pommodoro-clock.lua#L230-L235

```
01:33:26 msg_show Error executing lua callback: ...e/nvim/lazy/pommodoro-clock.nvim/lua/pommodoro-clock.lua:233: attempt to index field 'popup' (a nil value)
stack traceback:
...e/nvim/lazy/pommodoro-clock.nvim/lua/pommodoro-clock.lua:233: in function <...e/nvim/lazy/pommodoro-clock.nvim/lua/pommodoro-clock.lua:232>
```

To reproduce:

1. run `require('pommodoro-clock').start(some_key)`.
2. run `require('pommodoro-clock').close()`.
3. create a vsplit.

I believe that you should remove this `autocmd` after it's `close()`d.