空引用异常控制

Question

我遇到了一些错误，无法理解为什么！

[NullReferenceException: Object reference not set to an instance of an object.] 
header.Page_Load(Object sender, EventArgs e) +867 System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +51 System.Web.UI.Control.OnLoad(EventArgs e) +92 
IRIPL.Sys5B.Web.cWebPageComponent.OnLoad(EventArgs e) +39 System.Web.UI.Control.LoadRecursive() +54 
System.Web.UI.Control.LoadRecursive() +145 System.Web.UI.Control.LoadRecursive() +145 
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +772