I upgraded to Umbraco 7.3.4 and got a weird behavior. Whenerver I click on any template or stylsheet in Umbraco backoffice - it displays an error:

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: 

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: 


[NullReferenceException: Object reference not set to an instance of an object.]
   Umbraco.Web.UI.Umbraco.Settings.Views.EditView.OnLoad(EventArgs e) +943
   System.Web.UI.Control.LoadRecursive() +70
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3177

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34280

Printscreen attached.

Any clue what can be done to fix this?