The `kickoff_async` try block starts before `FlowStartedEvent` is emitted,
so an abort in the execution-start hooks, in input handling or in state
restore emitted a `flow_failed` with no opener, which pops an unrelated
scope and warns about an empty scope stack. The failure event is now
gated on the flow scope actually being open, either from this kickoff's
`flow_started` or from a restored deferred session scope.
7fd1997e8e