Occasional Ag and Rg issues

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- [ ] Question
- [x] Bug
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- [ ] Vim
- [x] Neovim

nvim version: v0.4.2

From time to time `Ag` and `Rg` commands stop working correctly. I've redefined them a bit to make file grepping a bit better: https://github.com/jwadolowski/dotfiles/blob/master/config/nvim/init.vim#L199-L228

The issue occurs in a rather random fashion whenever I do this:
* `Ctrl+G` to invoke customized `Ag`
* type something in
* press ENTER to open up given file

ENTER ends with the following error:

```
Error detected while processing function 30[30]..29_callback:
line 21:
Vim(call):E16: Invalid range: call s:fill_quickfix(list)
```

It gets worse right after that. I can still trigger `Ag` and look for strings, however whenever I press ENTER quicklist disappears, but file is not opened (no explicit error this time):

I've been using `Rg` for a couple of weeks, but experienced similar problems, so switched back to `Ag`.

The only way to make it work again is to restart nvim.