After upgrading to umbraco 7.4.3 from 6.1.8 I recieved an error:

Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

[NullReferenceException: Object reference not set to an instance of an object.]
   umbraco.library.GetXmlNodeByXPath(String xpathQuery) +15

[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) +0
   System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) +128
   System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) +146
   UmbracoExamine.LegacyLibrary.GetXmlNodeByXPath(String xpathQuery) +116
   UmbracoExamine.DataServices.UmbracoContentService.GetPublishedContentByXPath(String xpath) +12
   UmbracoExamine.BaseUmbracoIndexer.AddNodesToIndex(String xPath, String type) +30
   UmbracoExamine.BaseUmbracoIndexer.PerformIndexAll(String type) +1480
   UmbracoExamine.UmbracoContentIndexer.PerformIndexAll(String type) +156
   UmbracoExamine.BaseUmbracoIndexer.IndexAll(String type) +35
   UmbracoExamine.BaseUmbracoIndexer.PerformIndexRebuild() +112
   UmbracoExamine.UmbracoContentIndexer.RebuildIndex() +86
   Umbraco.Web.WebBootManager.<InitializeResolvers>b__f() +177
   Umbraco.Core.Sync.DatabaseServerMessenger.Initialize() +774
   Umbraco.Web.BatchedDatabaseServerMessengerStartup.ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext) +87
   Umbraco.Core.ApplicationEventHandler.OnApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext) +50
   Umbraco.Core.CoreBootManager.<Complete>b__a(IApplicationEventHandler x) +434
   Umbraco.Core.EnumerableExtensions.ForEach(IEnumerable`1 items, Action`1 action) +167
   Umbraco.Core.CoreBootManager.Complete(Action`1 afterComplete) +467
   Umbraco.Web.WebBootManager.Complete(Action`1 afterComplete) +167
   Umbraco.Core.UmbracoApplicationBase.StartApplication(Object sender, EventArgs e) +364

[HttpException (0x80004005): Exception has been thrown by the target of an invocation.]
   System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app) +544
   System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +186
   System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +172
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +402
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +343

[HttpException (0x80004005): Exception has been thrown by the target of an invocation.]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +579
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +112
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +716

There is a person that recieved the same error here, with no clear solution on how to fix it.