UI: Fix multithread-unsafe GetCurrentScene

The function `OBSBasic::GetCurrentScene()` is also called from the
graphics thread and access to a QListWidget object, which is not
multithread-safe.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f