Focusing VS Search (Ctrl+Q) causes an error dialog when no files are open

[olegtk]

See https://developercommunity.visualstudio.com/t/Quick-Search-gives-immediate-error-when-/1374751

We identified the issues is caused by NullReferenceException in DiffClipboardWithCodeCommand.OnBeforeQueryStatus() and then BaseCommand.OnBaseBeforeQueryStatus() showing an error dialog.

Please don't show any dialogs during QueryStatus calls, they are intended to be very fast and have no side effects except modifying command state.
Thanks!

Name

Throw(System.NullReferenceException) Object reference not set to an instance of an object.

• clr!EtwCallout
• clr!CoTemplate_zzpqhh
• clr!ETW::ExceptionLog::ExceptionThrown
• clr!COMPlusThrowCallback
• clr!Thread::StackWalkFramesEx
• clr!Thread::StackWalkFrames
• clr!CPFH_RealFirstPassHandler
• clr!CPFH_FirstPassHandler
• clr!_COMPlusFrameHandler
• clr!_COMPlusFrameHandlerRevCom
• ntdll!ExecuteHandler2
• ntdll!ExecuteHandler
• ntdll!KiUserExceptionDispatcher
• Kool.VsDiff!DiffClipboardWithCodeCommand.OnBeforeQueryStatus
  |+ Kool.VsDiff!BaseCommand.OnBaseBeforeQueryStatus
  ...

[heku]

Thanks for your suggestion and code fix, I'll merge it soon.