DefaultFocusHandler.OnGotFocus - Add browser null check
DefaultFocusHandler.OnGotFocus - Add browser null check

For the print dialog we appear to be passed a CefRefPtr<CefBrowser> reference which doesn't appear to trigger an OnAfterCreated call
so we have no reference to it. Looking at the `CefBrowser`, there's no real data of value, empty url, no frame information (just an invalid frame id)
so for now we'll just ignore it in DefaultFocusHandler, we don't want to handle for popups anyway

Resolves #2725

Livit/CefSharp,Livit/CefSharp,Livit/CefSharp,Livit/CefSharp