EntityScriptServer should wait for script engine to finish when stopping

Script engine will still running and using EntityEditPacketSender,
which is owned by EntityScriptServer, which was destroyed.
https://highfidelity.manuscript.com/f/cases/11071/

assignment-client/src/scripts/EntityScriptServer.cpp

@@ -476,6 +476,7 @@ void EntityScriptServer::clear() {
         // do this here (instead of in deleter) to avoid marshalling unload signals back to this thread
         _entitiesScriptEngine->unloadAllEntityScripts();
         _entitiesScriptEngine->stop();
+        _entitiesScriptEngine->waitTillDoneRunning();
     }
 
     _entityViewer.clear();