DebuggerEngine state change at shutdown

When ending a debugging session, the slave engine (QmlEngine) tries to switch from "EngineRunRequested" directly to "InferiorShutdownRequested" without going through "InferiorRunOK" and "InferiorStopRequested". This produces a warning because it's not a valid state change. Despite the warning, the shutdown process continues normally.

Investigate causes and possible solution.