Handle the case where we have a p2p ref that we don't have project info for

src/VisualStudio/Core/Def/Implementation/ProjectSystem/VisualStudioProjectTracker_IVsSolutionLoadEvents.cs

@@ -158,7 +158,12 @@ private void OutputToOutputWindow(string message)
                 return null;
             }
 
-            var projectInfo = allProjectInfos[projectFilename];
+            DeferredProjectInformation projectInfo;
+            if (!allProjectInfos.TryGetValue(projectFilename, out projectInfo))
+            {
+                return null;
+            }
+
             var commandLineParser = _workspace.Services.GetLanguageServices(languageName).GetService<ICommandLineParserService>();
             var projectDirectory = Path.GetDirectoryName(projectFilename);
             var commandLineArguments = commandLineParser.Parse(