CS0122: 'System.Configuration.StringUtil' is inaccessible due to its protection level

In Visual Studio

File - Open - Website
GoTo: C:\Windows\Microsoft.NET\Framework\v4.0.30319\ASP.NETWebAdminFiles
Open
Now open App_Code\WebAdminPage.cs
GoTo Line 989
Comment out the current text and paste

string appId = (String.Concat(appPath, appPhysPath).GetHashCode()).ToString("x", CultureInfo.InvariantCulture);