[Bug] crash due to nil target in sync_scroll

[dlyongemallo]

Description

Occasionally, when exiting neovim with DiffviewOpen, there is an error message: `diffview.nvim/lua/diffview/async.lua:187: The coroutine failed with this message: ... diffview.nvim/lua/diffview/scene/layout.lua:308: attempt to index local 'target' (a nil value)'.

Here's the entire error message:

Expected behavior

Neovim exits normally.

Actual behavior

Diffview crashes and neovim displays an error message.

Steps to reproduce

Unfortunately, the issue isn't deterministically reproducible. So the following steps are necessary to reproduce, but possibly insufficient.

1. start neovim
2. use DiffviewOpen with some commit
3. do stuff, then exit

Sometimes, neovim exits normally, and sometimes, the above error appears.

Health check

Output of :checkhealth diffview

diffview: require("diffview.health").check()

Checking plugin dependencies ~
- OK nvim-web-devicons installed.

Checking VCS tools ~
- The plugin requires at least one of the supported VCS tools to be valid.
- OK Git found.
- OK Git is up-to-date. (2.43.0)
- WARNING Configured `hg_cmd` is not executable: 'hg'

Log info

No relevant details from DiffviewLog.

Neovim version

NVIM v0.10.3
Build type: RelWithDebInfo
LuaJIT 2.1.1713484068

Operating system and version

Linux 6.8.0-51-generic x86_64 GNU/Linux

Minimal config

No response