b2950de4a2
Fixes: #136842, #136645
Caused by 76d6d169ba
When multiple objects are selected to mark as assets, preview generation
job of previous ID in the for loop is cleared by `ED_preview_kill_jobs`
inside `generate_preview`. To fix this, check if `id->preview` exists then
clear the preview job if exists inside the new function
`ED_preview_kill_jobs_for_id`.
Solution proposed by @JulianEisel. I came up with idea to use `wm_job_find`
Pull Request: https://projects.blender.org/blender/blender/pulls/136918