Data Explorer Page Error

Type: "System.NullReferenceException"

ErrorMessage: "System.NullReferenceException: Object reference not set to an instance of an object.
   at variables.Page_Init(Object sender, EventArgs e) in C:\projects\NDECore2015\NDE3\variables.aspx.cs:line 401
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Web.UI.Control.OnInit(EventArgs e)
   at System.Web.UI.Page.OnInit(EventArgs e)
   at System.Web.UI.Control.InitRecursive(Control namingContainer)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)"