COMP: fix compiling errors on linux/gcc

Source/Rendering/VTKRenderer/imstkVTKRenderer.h

@@ -168,9 +168,9 @@ protected:
     std::vector<vtkOpenVRCameraPose> m_camPos;
 #endif
 
-    vtkSmartPointer<vtkChartXY>      m_timeTableChart      = nullptr;
-    vtkSmartPointer<vtkContextActor> m_timeTableChartActor = nullptr;
-    vtkSmartPointer<vtkTable> m_timeTable = nullptr;
+    vtkSmartPointer<vtkChartXY>      m_timeTableChart;
+    vtkSmartPointer<vtkContextActor> m_timeTableChartActor;
+    vtkSmartPointer<vtkTable> m_timeTable;
     vtkPlotBar* m_timeTablePlot = nullptr;
     int m_timeTableIter = 0;
 };

Source/Scene/imstkScene.h

@@ -282,7 +282,8 @@ protected:
     NamedMap<Light>                 m_lightsMap;
     std::shared_ptr<IBLProbe>       m_globalIBLProbe = nullptr;
     std::shared_ptr<Camera>         m_camera = std::make_shared<Camera>();
-    std::shared_ptr<CollisionGraph> m_collisionGraph = std::make_shared<CollisionGraph>();
+    // std::shared_ptr<CollisionGraph> m_collisionGraph = std::make_shared<CollisionGraph>();
+    std::shared_ptr<CollisionGraph> m_collisionGraph = nullptr; 
     std::vector<std::shared_ptr<SceneObjectControllerBase>> m_objectControllers; ///> List of object controllers
     std::vector<std::shared_ptr<CameraController>> m_cameraControllers;          ///> List of camera controllers
     std::unordered_map<std::string, std::thread>   m_threadMap;                  ///>

Source/SimulationManager/VTKRenderer/imstkVTKViewer.h

@@ -115,7 +115,7 @@ protected:
     std::shared_ptr<VTKInteractorStyle> m_vtkInteractorStyle;
     bool m_enableVR;
     std::string m_windowName = "imstk";
-    vtkSmartPointer<vtkCallbackCommand> timerCallbackCommand = nullptr;
+    vtkSmartPointer<vtkCallbackCommand> timerCallbackCommand;
 
 #ifdef iMSTK_ENABLE_VR
     vtkSmartPointer<OpenVRCommand> m_openVRCommand;