Steps to reproduce:
1. setting `vim.highlight.on_yank`
```
vim.api.nvim_create_autocmd({ "TextYankPost" }, {
pattern = { "*" },
callback = function()
vim.highlight.on_yank({ timeout = 200 })
end,
})
```
2. repeat typing `yeye` ...
3. causes the following error.
```
Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x01e96970 is already closing
stack traceback:
[C]: in function 'close'
vim/_editor.lua: in function ''
vim/_editor.lua: in function <vim/_editor.lua:0>
```
📝 Test result before fix:
[----------] Global test environment setup.
[----------] Running tests from test/functional/lua/highlight_spec.lua
[ RUN ] vim.highlight.on_yank does not show errors even if buffer is wiped before timeout: 15.07 ms OK
[ RUN ] vim.highlight.on_yank does not show errors even if executed between timeout and clearing highlight: 15.07 ms ERR
test/helpers.lua:73: Expected objects to be the same.
Passed in:
(string) 'Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x02025260 is already closing
stack traceback:
[C]: in function 'close'
vim/_editor.lua: in function ''
vim/_editor.lua: in function <vim/_editor.lua:0>'
Expected:
(string) ''
Resolve an issue with deferred clearing of highlight failing if the
buffer is deleted before the timeout by checking whether the
buffer is valid first.
