Autoclose not working

**Describe the bug**
I have enabled ```mkdp_auto_close```. Yet when I do ```:MarkdownPreviewStop``` or when I close the buffer, the preview window doesn't close

**To Reproduce**
Steps to reproduce the behavior:

I have set ```mkdp_browserfunc``` to the following
```
function OpenURL(url)
execute 'silent' '!qutebrowser' '--target' 'private-window' shellescape(a:url)
endfunction
```

**Expected behavior**
The preview window to close when closing the buffer.

**Desktop (please complete the following information):**
- OS: 5.11.7-arch1-1