Geant4MT: Fix closing sink in MT mode

In the MT mode FairRunSim cannot close the sinks,
only FairMCApplication can.

Author: karabowi

fairroot/base/sim/FairMCApplication.cxx

@@ -477,6 +477,10 @@ void FairMCApplication::FinishRunOnWorker()
     LOG(debug) << "FairMCApplication::FinishRunOnWorker: ";
 
     FinishRun();
+
+    // Close the sink on workers so that the execution of main FairRunSim->Run()
+    // finishes with the output files properly closed.
+    fRootManager->CloseSink();
 }
 
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