If during the report rendering stage, one or more of the required entities in the File storage were removed/deleted by third-party software, it will prevent the report from finishing the rendering and throws a NullRefereceException:
System.NullReferenceException: Object reference not set to an instance of an object.
at Telerik.Reporting.Services.Engine.Document.AddClientToolTipsResource(IList toolTips)
at Telerik.Reporting.Services.Engine.Document.FinishRendering(ReportRendererResult result)
at Telerik.Reporting.Services.Engine.ReportEngine.RenderReportDocument(String pendingRenderingId)
at Telerik.Reporting.Services.Engine.TaskQueue.WorkerThread.ProcessItems()
at Telerik.Reporting.Services.Engine.TaskQueue.WorkerThread.ThreadProc()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()]