`undo-tree` is not displayed in a popup until second launch

### What did you expect to happen?

Launching `undo-tree-visualize` should open a thin popup on the left side

### What actually happened?

When `undo-tree-visualize` is launched for the first time in a session, it will be displayed in the classic buffer. But, from the second launched onwards, it will be displayed as a proper popup.

It seems to be related to an old issue (https://github.com/hlissner/doom-emacs/issues/5617#issuecomment-968958532), but the it still persists in the latest Doom version.

### Describe your attempts to resolve the issue

I tried applying @vqhuy's fix (https://github.com/vqhuy/doom-emacs/commit/5bf696bd76edbcf8ee8b90f5ea5bf5be58415296), and it fixed the original issue, but introduced a new one. When the visualizer is opened, a diff buffer (which isn't the default Doom behavior) is also opened alongside the popup. When the popup is closed, the diff buffer will be left dangling open.

### Steps to reproduce

1. Activate `undo-tree` with `:emacs (undo +tree)`
2. Sync and restart Doom
3. Open a text file, and do some edits
4. Execute `undo-tree-visualize`, it will open in the classic buffer
5. Execute it again, it will no open as a popup

### System Information

https://pastebin.com/SaNXuK6e