Document VisualStudioWorkspace.GetHierarchy can return null

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

@@ -78,6 +78,11 @@ protected override void OnDocumentClosing(DocumentId documentId)
             }
         }
 
+        /// <summary>
+        /// Returns the hierarchy for a given project. 
+        /// </summary>
+        /// <param name="projectId">The <see cref="ProjectId"/> for the project.</param>
+        /// <returns>The <see cref="IVsHierarchy"/>, or null if the project doesn't have one.</returns>
         public abstract IVsHierarchy GetHierarchy(ProjectId projectId);
         public abstract string GetFilePath(DocumentId documentId);