Add Content throws an error

I am running a Sitefinity trial and having trouble when I tried to add any Content widget. I keep getting the error below when I try to edit:

Server Error in '/' Application.Value cannot be null.
Parameter name: sourceDescription: 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.ArgumentNullException: Value cannot be null.
Parameter name: source

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace: 

[ArgumentNullException: Value cannot be null.
Parameter name: source]
System.Linq.Enumerable.Select(IEnumerable`1 source, Func`2 selector) +6143288
Telerik.Sitefinity.Data.ManagerExtensions.GetSiteProviders(IManager manager, Guid siteId) +542
Telerik.Sitefinity.Data.ManagerBase`1.GetContextProviders() +60
Telerik.Sitefinity.Web.UI.ProvidersSelector.get_Providers() +209
Telerik.Sitefinity.Web.UI.ProvidersSelector.OnPreRender(EventArgs e) +57
System.Web.UI.Control.PreRenderRecursiveInternal() +113
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4297

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18408

 What am I missing here?

Hi Johan,

Thank you for the resources provided.

The error trace below is not sufficient in order to identify the source of the issue. Can you please provide some additional information - when did the error start to occur, does occur on every page with every content widget, or on a specified page and some content widgets?

Additionally, if you can attach your Error.log file located in your App_Data/Sitefinity/Logs folder to your reply for me to examine, this will help us arrive at a solution as well.

Your cooperation is appreciated. Looking forward to your response.

Regards,
Ivan D. Dimitrov
Telerik