On Visual Studio 2013 RTM/Windows 8.1 with VA 10.8.2001 the IDE shows an message box with an exception warning in an extension when the first file is opened in the editor. It's a fresh install with VAX only.
Activity.xml shows the following entries:
System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt. bei t.TextViewCreated(IWpfTextView textView) bei
Microsoft.VisualStudio.Text.Editor.Implementation.WpfTextView.<>c__DisplayClassc.<BindContentTypeSpecificAssets>b__9() bei
Microsoft.VisualStudio.Text.Utilities.GuardedOperations.CallExtensionPoint(Object errorSource, Action call)
The color 'Popup' in category 'de7b1121-99a4-4708-aedf-15f40c9b332f' does not exist.
Maybe it's worth to mention that the file is first colored by VA before the message box comes up.