Fixed NullReferenceException in DoClose (#1980)

* Fixed NullReferenceException in DoClose

Author: merceyz

CefSharp.Core/Internals/ClientAdapter.cpp

@@ -200,9 +200,12 @@ namespace CefSharp
 
             if (handler != nullptr)
             {
-                auto browserWrapper = GetBrowserWrapper(browser->GetIdentifier(), browser->IsPopup());
-
-                return handler->DoClose(_browserControl, browserWrapper);
+                //By this point it's possible IBrowser references have been disposed
+                //Rather than attempting to rework the rather complex closing logic
+                //It's easier to pass in a new wrapper and dispose it straight away
+                CefSharpBrowserWrapper browserWrapper(browser);
+                
+                return handler->DoClose(_browserControl, %browserWrapper);
             }
 
             return false;