'Activate feature' step error :Object Reference not set to an instance of an object

I created a custom timer job and implemented it through a feature event in sharepoint.

when i tried to activate it, its showing this particular error:

Object reference not set to an instance of an object.   at TimerJobExample.MyFeatureReceiver.FeatureActivated(SPFeatureReceiverProperties properties) in C:\Documents and Settings\admin-shuklag.INFRADEV\My Documents\Visual Studio 2008\Projects\TimerJobExample\TimerJobExample\MyFeatureReceiver.cs:line 22 
   at Microsoft.SharePoint.SPFeature.DoActivationCallout(Boolean fActivate, Boolean fForce) 
   at Microsoft.SharePoint.SPFeature.Activate(SPSite siteParent, SPWeb webParent, SPFeaturePropertyCollection props, Boolean fForce) 
   at Microsoft.SharePoint.SPFeatureCollection.AddInternal(Guid featureId, SPFeaturePropertyCollection properties, Boolean force, Boolean fMarkOnly) 
   at Microsoft.SharePoint.SPFeatureCollection.Add(Guid featureId) 
   at Microsoft.SharePoint.WebControls.FeatureActivator.BtnActivateFeature_Click(Object objSender, EventArgs evtargs) 
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e) 
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) 
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) 
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) 
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) 
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) 

The error line is highlighted below

public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {

        SPSite site = properties.Feature.Parent as SPSite;

        // make sure the job isn't already registered


//error line
      **foreach (SPJobDefinition job in site.WebApplication.JobDefinitions)**

        {

            if (job.Name == List_JOB_NAME)

                job.Delete();

        }



        SampleTimer listLoggerJob = new SampleTimer(List_JOB_NAME, site.WebApplication);

        SPMinuteSchedule schedule = new SPMinuteSchedule();

        schedule.BeginSecond = 0;

        schedule.EndSecond = 59;

        schedule.Interval = 5;

        listLoggerJob.Schedule = schedule;

        listLoggerJob.Update();

    }

what could be the problem?

The problem was with the scope of the feature. It was set to Web, and for a feature to be activated at this level, u need an admin account or an account with elevated privileges.

You can either add the application pool id to farm admin accounts or run the code with elevated rights.