Server Error in '/' Application.

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Line 869:                    HttpContext.Current.Cache.Add("alternative_" + Tools.Query_String("liste","modele").ToString(), valalternative, null, DateTime.MaxValue, TimeSpan.FromHours(24), System.Web.Caching.CacheItemPriority.Default, null);
Line 870:                }
Line 871:                alternative.Text = HttpContext.Current.Cache.Get("alternative_" + Tools.Query_String("liste","modele").ToString()).ToString(); 
Line 872:            }
Line 873:			if (Tools.Query_String("liste","modele") != null || Tools.Query_String("liste","marque") != null){ // si le champ modele est rensigné on fait les liens intesite AE

[NullReferenceException: Object reference not set to an instance of an object.]
   recherche.SearchLucene() in d:\Web321\V4_TEST\Occasion\Annonces-auto\Result.aspx.cs:871
   recherche.Page_Load(Object sender, EventArgs e) in d:\Web321\V4_TEST\Occasion\Annonces-auto\Result.aspx.cs:117
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +34
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   System.Web.UI.Control.LoadRecursive() +47
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1061