Aircraft-model subsystem behaves better

- make shutdown logic more robust
- shutdown is no longer special cased in globals.

src/Main/fg_init.cxx

@@ -894,7 +894,7 @@ void fgCreateSubsystems(bool duringReset) {
         globals->add_subsystem("events", globals->get_event_mgr(), SGSubsystemMgr::DISPLAY);
     }
 
-    globals->add_subsystem("aircraft-model", new FGAircraftModel, SGSubsystemMgr::DISPLAY);
+    globals->add_new_subsystem<FGAircraftModel>(SGSubsystemMgr::DISPLAY);
     globals->add_new_subsystem<FGModelMgr>(SGSubsystemMgr::DISPLAY);
 
     globals->add_new_subsystem<FGViewMgr>(SGSubsystemMgr::DISPLAY);

src/Main/globals.cxx

@@ -215,8 +215,6 @@ FGGlobals::~FGGlobals()
     subsystem_mgr->shutdown();
     subsystem_mgr->unbind();
 
-    subsystem_mgr->remove("aircraft-model");
-
     subsystem_mgr->remove(FGTileMgr::subsystemName());
 
     osg::ref_ptr<osgViewer::Viewer> vw(renderer->getViewer());

src/Model/acmodel.cxx

@@ -139,15 +139,18 @@ FGAircraftModel::reinit()
 void
 FGAircraftModel::shutdown()
 {
-  if (!_aircraft.get()) {
+    FGScenery* scenery = globals->get_scenery();
-    return;
-  }
 
-  osg::Node* node = _aircraft->getSceneGraph();
+    if (_aircraft.get()) {
-  globals->get_scenery()->get_aircraft_branch()->removeChild(node);
+        if (scenery && scenery->get_aircraft_branch()) {
+            scenery->get_aircraft_branch()->removeChild(_aircraft->getSceneGraph());
+        }
+    }
 
     if (_interior.get()) {
-        globals->get_scenery()->get_interior_branch()->removeChild(_interior->getSceneGraph());
+        if (scenery && scenery->get_interior_branch()) {
+            scenery->get_interior_branch()->removeChild(_interior->getSceneGraph());
+        }
     }
 
     _aircraft.reset();

src/Model/acmodel.hxx

@@ -34,6 +34,7 @@ public:
   virtual SGModelPlacement * get3DModel() { return _aircraft.get(); }
   virtual SGVec3d& getVelocity() { return _velocity; }
 
+    static const char* subsystemName() { return "aircraft-model"; }
 private:
   void deinit ();