Remove duplicate project existence check

CheckContainsProject asserts the project exists, no need to duplicate
it.

src/Workspaces/Core/Portable/Workspace/Solution/SolutionState.cs

@@ -1217,12 +1217,7 @@ public SolutionState AddDocuments(ImmutableArray<DocumentInfo> documentInfos)
             foreach (var documentInfosInProject in documentInfosByProjectId)
             {
                 CheckContainsProject(documentInfosInProject.Key);
-                var oldProject = this.GetProjectState(documentInfosInProject.Key);
-
-                if (oldProject == null)
-                {
-                    throw new InvalidOperationException(string.Format(WorkspacesResources._0_is_not_part_of_the_workspace, documentInfosInProject.Key));
-                }
+                var oldProject = this.GetProjectState(documentInfosInProject.Key)!;
 
                 var newDocumentStatesForProjectBuilder = ArrayBuilder<T>.GetInstance();