fix crash due to race in build synchronization

src/Features/Core/Portable/Diagnostics/EngineV2/DiagnosticIncrementalAnalyzer.ProjectState.cs

@@ -200,7 +200,17 @@ public async Task SaveAsync(Project project, AnalysisResult result)
                 foreach (var documentId in result.DocumentIds)
                 {
                     var document = project.GetDocument(documentId);
-                    Contract.ThrowIfNull(document);
+                    if (document == null)
+                    {
+                        // it can happen with build synchronization since, in build case, 
+                        // we don't have actual snapshot (we have no idea what sources out of proc build has picked up)
+                        // so we might be out of sync.
+                        // example of such cases will be changing anything about solution while building is going on.
+                        // it can be user explict actions such as unloading project, deleting a file, but also it can be 
+                        // something project system or roslyn workspace does such as populating workspace right after
+                        // solution is loaded.
+                        continue;
+                    }
 
                     await SerializeAsync(serializer, document, document.Id, _owner.SyntaxStateName, GetResult(result, AnalysisKind.Syntax, document.Id)).ConfigureAwait(false);
                     await SerializeAsync(serializer, document, document.Id, _owner.SemanticStateName, GetResult(result, AnalysisKind.Semantic, document.Id)).ConfigureAwait(false);