Error caught in Application_Error event

Question

i have caught this error on server. but i m not getting it on local. how to get solution for the below problem.  Error caught in Application_Error event Error in: http://cds.smartclassonline.com/Lessons/Elements/ReeditElements.aspx DateTime: 4/13/2012 10:10:54 AM User ID: Content Editor IP Address: 202.54.232.162 / 202.54.232.162 Error Message: Exception of type 'System.Web.HttpUnhandledException' was thrown. Error Message: System.Web.HttpUnhandledException (0x80004005): Exception of type 'System.Web.HttpUnhandledException' was thrown. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Infragistics.Web.UI.ListControls.WebDropDown.get_SelectedValue() at UserControls_SchoolInfo_BookFilter.get_BookID() in c:\WWW\cds\smartclasscds\UserControls\SchoolInfo\BookFilter.ascx.cs:line 99 at UserControls_Lessons_Elements_PendingElementList.BindData() in c:\WWW\cds\smartclasscds\UserControls\Lessons\Elements\PendingElementList.ascx.cs:line 48 at UserControls_Lessons_Elements_PendingElementList.btnSearch_Click(Object sender, EventArgs e) in c:\WWW\cds\smartclasscds\UserControls\Lessons\Elements\PendingElementList.ascx.cs:line 39 at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.lessons_elements_reeditelements_aspx.ProcessRequest(HttpContext context) in c:\Temp\root\4f824cfb\80ddd9d2\App_Web_h2f5tyom.12.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Error Message: at System.Web.UI.Page.HandleError(Exception e) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) at System.Web.UI.Page.ProcessRequest() at System.Web.UI.Page.ProcessRequest(HttpContext context) at ASP.lessons_elements_reeditelements_aspx.ProcessRequest(HttpContext context) in c:\Temp\root\4f824cfb\80ddd9d2\App_Web_h2f5tyom.12.cs:line 0 at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) Error Type: System.Web.HttpUnhandledException