Avoid unexpected panic in graceful manager (#29629)

Avoid unexpected panic in graceful manager (#29629)

There is a fundamental design problem of the "manager" and the "wait
group".
If nothing has started, the "Wait" just panics: sync: WaitGroup is
reused before previous Wait has returned
There is no clear solution besides a complete rewriting of the "manager"

If there are some mistakes in the app.ini, end users would just see the
"panic", but not the real error messages. A real case: #27643

This PR is just a quick fix for the annoying panic problem.