Problem in starting Sharepoint workflow

I have a custom workflow that I am starting through code.The following code I am using to start a workflow.

DeleteWorkflowTasks(properties.ListItem.ID);
                        Thread thread = new Thread(delegate() { StartApprovalWorkflow(); });
                        thread.Start();

private void StartApprovalWorkflow()
        {
            try
            {
                SPSecurity.RunWithElevatedPrivileges(delegate()
                    {
                        try
                        {
                            using (SPSite elevatedSite = new SPSite(siteID))
                            {
                                using (SPWeb elevatedWeb = elevatedSite.OpenWeb(webID))
                                {

                                    SPList calendarList = elevatedWeb.Lists[listID];
SPWorkflowAssociation workflowAssociation = calendarList.WorkflowAssociations.GetAssociationByName(currentWorkflowName, System.Threading.Thread.CurrentThread.CurrentCulture);
                                    elevatedSite.WorkflowManager.StartWorkflow(itemBeforeUpdate, workflowAssociation, workflowAssociation.AssociationData);
                                }
                            }
                        }
                        catch (SPException ex)
                        {
                                                    }
                        catch (Exception ex)
                        {
                                                    }
                    });
            }
            catch (SPException ex)
            {

            }
            catch (Exception ex)
            {
            }
        }

This code is attached with ItemUpdated eventhandler code.Whenever I update an item , it cancels the workflow which is already attached and creates a new workflow for the same item.Though it is working fine however sometimes the workflow is not getting started however the previous workflow is getting cancelled. The stack trace which I am getting is as follows.

Object reference not set to an instance of an object.   at Microsoft.SharePoint.Library.SPRequestInternalClass.OpenWeb(String bstrUrl, String& pbstrServerRelativeUrl, String& pbstrTitle, String& pbstrDescription, Guid& pguidID, String& pbstrRequestAccessEmail, UInt32& pwebVersion, Guid&开发者_JAVA百科amp; pguidScopeId, UInt32& pnAuthorID, UInt32& pnLanguage, UInt32& pnLocale, UInt16& pnTimeZone, Boolean& bTime24, Int16& pnCollation, UInt32& pnCollationLCID, Int16& pnCalendarType, Int16& pnAdjustHijriDays, Int16& pnAltCalendarType, Boolean& pbShowWeeks, Int16& pnFirstWeekOfYear, UInt32& pnFirstDayOfWeek, Int16& pnWorkDays, Int16& pnWorkDayStartHour, Int16& pnWorkDayEndHour, Int16& pnMeetingCount, Int32& plFlags, Boolean& bConnectedToPortal, String& pbstrPortalUrl, String& pbstrPortalName, Int32& plWebTemplateId, Int16& pnProvisionConfig, String& pbstrDefaultTheme, String& pbstrDefaultThemeCSSUrl, String& pbstrAlternateCSSUrl, String& pbstrCustomizedCssFileList, String& pbstrCustomJSUrl, String& pbstrAlternateHeaderUrl, String& pbstrMasterUrl, String& pbstrCustomMasterUrl, String& pbstrSiteLogoUrl, String& pbstrSiteLogoDescription, Object& pvarUser, Boolean& pvarIsAuditor, UInt64& ppermMask, Boolean& bUserIsSiteAdmin, Boolean& bHasUniquePerm, Guid& pguidUserInfoListID, Guid& pguidUniqueNavParent, Int32& plSiteFlags, DateTime& pdtLastContentChange, DateTime& pdtLastSecurityChange, String& pbstrWelcomePage)
   at Microsoft.SharePoint.Library.SPRequest.OpenWeb(String bstrUrl, String& pbstrServerRelativeUrl, String& pbstrTitle, String& pbstrDescription, Guid& pguidID, String& pbstrRequestAccessEmail, UInt32& pwebVersion, Guid& pguidScopeId, UInt32& pnAuthorID, UInt32& pnLanguage, UInt32& pnLocale, UInt16& pnTimeZone, Boolean& bTime24, Int16& pnCollation, UInt32& pnCollationLCID, Int16& pnCalendarType, Int16& pnAdjustHijriDays, Int16& pnAltCalendarType, Boolean& pbShowWeeks, Int16& pnFirstWeekOfYear, UInt32& pnFirstDayOfWeek, Int16& pnWorkDays, Int16& pnWorkDayStartHour, Int16& pnWorkDayEndHour, Int16& pnMeetingCount, Int32& plFlags, Boolean& bConnectedToPortal, String& pbstrPortalUrl, String& pbstrPortalName, Int32& plWebTemplateId, Int16& pnProvisionConfig, String& pbstrDefaultTheme, String& pbstrDefaultThemeCSSUrl, String& pbstrAlternateCSSUrl, String& pbstrCustomizedCssFileList, String& pbstrCustomJSUrl, String& pbstrAlternateHeaderUrl, String& pbstrMasterUrl, String& pbstrCustomMasterUrl, String& pbstrSiteLogoUrl, String& pbstrSiteLogoDescription, Object& pvarUser, Boolean& pvarIsAuditor, UInt64& ppermMask, Boolean& bUserIsSiteAdmin, Boolean& bHasUniquePerm, Guid& pguidUserInfoListID, Guid& pguidUniqueNavParent, Int32& plSiteFlags, DateTime& pdtLastContentChange, DateTime& pdtLastSecurityChange, String& pbstrWelcomePage)
   at Microsoft.SharePoint.SPWeb.InitWeb()
   at Microsoft.SharePoint.SPWeb.get_UserInfoListId()
   at Microsoft.SharePoint.SPListItem.CalculateEffectivePermMask(SPBasePermissions permIn)
   at Microsoft.SharePoint.SPListItem.get_EffectiveBasePermissions()
   at Microsoft.SharePoint.SPListItem.DoesUserHavePermissions(SPBasePermissions permissionMask)
   at Microsoft.SharePoint.Workflow.SPWorkflowManager.StartWorkflow(SPListItem item, SPWorkflowAssociation association, String eventData, Boolean isAutoStart)
   at Microsoft.SharePoint.Workflow.SPWorkflowManager.StartWorkflow(SPListItem item, SPWorkflowAssociation association, String eventData)

Appreciate any help on this.

Complex objects you reference inside an elevated context MUST be created inside that context

So this:

elevatedSite.WorkflowManager.StartWorkflow(itemBeforeUpdate, ...

Needs to be changed to something like this:

elevatedItem = calendarList.GetItemById(itemBeforeUpdate.Id)
elevatedSite.WorkflowManager.StartWorkflow(elevatedItem, ...