I am stuck with upgrading DNN from Version 6.x to 7.4

I am using Ifnity URL Master V2.4.4 with DNN 6. Before upgrading DNN to 7.4, I have first upgraded Ifinity URL Master from 2.4 to 2.8.3 and then upgraded DNN to 7.4. while doing so I am getting below error on Default.aspx load.

 500 Internal Server Error
    The requested Url does not return any valid content.

    Administrators
    Change this message by configuring a specific 404 Error Page or Url for this website.

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

    Stack Trace:

at DotNetNuke.Common.Globals.IsEditMode() at DotNetNuke.Entities.Tabs.TabModulesController.GetModules(TabInfo tab) at DotNetNuke.Entities.Tabs.TabModulesController.GetTabModules(TabInfo tab) at DotNetNuke.Entities.Tabs.TabInfo.get_Modules() at iFinity.DNN.Modules.UrlMaster.Entities.TabInfoEx.Clone(TabInfo tab) at iFinity.DNN.Modules.UrlMaster.Data.SqlDataProvider.GetTabs(Int32 portalId, Boolean includeAdminTabs, Boolean includeHostTabs, Dictionary2& customAliasTabs) at iFinity.DNN.Modules.UrlMaster.UrlMasterController.GetTabs(Int32 portalId, Boolean includeStdUrls, PortalSettings portalSettings, FriendlyUrlSettings settings, Dictionary2& customAliasTabsForPortal) at iFinity.DNN.Modules.UrlMaster.UrlMasterController.GetTabs(Int32 portalId, Boolean includeStdUrls, FriendlyUrlSettings settings, Dictionary2& customAliasTabs) at iFinity.DNN.Modules.UrlMaster.TabDictController.BuildTabDictionary(PathSizes& pathSizes, FriendlyUrlSettings settings, Int32 buildPortalId, ThreadSafeDictionary2 tabDict, Hashtable& homePageSkins, ThreadSafeDictionary`2& portalTabPathDictionary, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.TabDictController.FetchTabDictionaryLockable(Int32 portalId, Int32& minTabPathDepth, Int32& maxTabPathDepth, Int32& minAliasPathDepth, Int32& maxAliasPathDepth, FriendlyUrlSettings settings, Boolean forceRebuild, Boolean bypassCache, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.TabDictController.FetchTabDictionary(Int32 portalId, Int32& minTabPathDepth, Int32& maxTabPathDepth, Int32& minAliasPathDepth, Int32& maxAliasPathDepth, FriendlyUrlSettings settings, Boolean forceRebuild, Boolean bypassCache, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.RewriteController.GetTabFromDictionary(String url, FriendlyUrlSettings settings, UrlAction result, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.RewriteController.IdentifyByTabPathEx(String absoluteUri, String queryString, UrlAction result, FriendlyUrlSettings settings, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.UrlRewriteModule.CheckForRewrite(String fullUrl, String querystring, UrlAction result, Boolean useFriendlyUrls, NameValueCollection queryStringCol, FriendlyUrlSettings settings, Boolean& isPhysicalResource, Guid parentTraceId) at iFinity.DNN.Modules.UrlMaster.UrlRewriteModule.ProcessRequest(HttpContext context, HttpRequest request, HttpServerUtility Server, HttpResponse response, Boolean useFriendlyUrls, String requestType, Uri requestUri, UrlAction result, NameValueCollection queryStringCol, FriendlyUrlSettings settings, Boolean allowSettingsChange, Guid parentTraceId)

I am not sure cause of this error and not sure how to forward in this. Please help me resolve this issue.