未将对象引用设置到对象的实例

  #region Overridden Class Methods

        /// <summary>
        /// Occurs when this command is created
        /// </summary>
        /// <param name="hook">Instance of the application</param>
        public override void OnCreate(object hook)
        {
            if (hook == null)
                return;

            m_application = hook as IApplication;

            //Disable if it is not ArcScene
            //if (hook is ISxApplication)
            //    base.m_enabled = true;
            //else
                base.m_enabled = false;

            // TODO:  Add other initialization code
            UID cmdUID = new UIDClass();
            cmdUID.Value = "esriArcScene.SxAddDataCommand";

    **        ICommandBars docBars = m_application.Document.CommandBars;**
            m_xoomCommand = docBars.Find(cmdUID, false, false);

            //Enable only when the delegate command is available
            base.m_enabled = m_xoomCommand != null;

        }

        /// <summary>
        /// Occurs when this command is clicked
        /// </summary>
        public override void OnClick()
        {
            // TODO: Add AddData.OnClick implementation
            m_xoomCommand.Execute();

        }

        #endregion

问题出现在加粗的部分

m_application
m_application.Document
m_application.Document.CommandBars
这三个变量哪个是null?