adds comments, fixes null check.

src/Umbraco.Web.Common/ApplicationBuilder/IUmbracoApplicationBuilderContext.cs

@@ -2,14 +2,32 @@ using System;
 
 namespace Umbraco.Cms.Web.Common.ApplicationBuilder
 {
-
+    /// <summary>
+    /// The context object used during 
+    /// </summary>
     public interface IUmbracoApplicationBuilderContext : IUmbracoApplicationBuilderServices
     {
+        /// <summary>
+        /// Called to include the core umbraco middleware.
+        /// </summary>
         void UseUmbracoCoreMiddleware();
 
+        /// <summary>
+        /// Manually runs the <see cref="IUmbracoPipelineFilter"/> pre pipeline filters
+        /// </summary>
         void RunPrePipeline();
+
+        /// <summary>
+        /// Manually runs the <see cref="IUmbracoPipelineFilter "/> post pipeline filters
+        /// </summary>
         void RunPostPipeline();
 
+        /// <summary>
+        /// Called to include all of the default umbraco required middleware.
+        /// </summary>
+        /// <remarks>
+        /// If using this method, there is no need to use <see cref="UseUmbracoCoreMiddleware"/>
+        /// </remarks>
         void RegisterDefaultRequiredMiddleware();
     }
 }

src/Umbraco.Web.Common/ApplicationBuilder/IUmbracoApplicationBuilderServices.cs

@@ -4,6 +4,9 @@ using Umbraco.Cms.Core.Services;
 
 namespace Umbraco.Cms.Web.Common.ApplicationBuilder
 {
+    /// <summary>
+    /// Services used during the Umbraco building phase.
+    /// </summary>
     public interface IUmbracoApplicationBuilderServices
     {
         IApplicationBuilder AppBuilder { get; }

src/Umbraco.Web.Common/ApplicationBuilder/UmbracoApplicationBuilder.cs

@@ -9,17 +9,20 @@ using Umbraco.Extensions;
 namespace Umbraco.Cms.Web.Common.ApplicationBuilder
 {
     /// <summary>
-    /// A builder to allow encapsulating the enabled endpoints in Umbraco
+    /// A builder used to enable middleware and endpoints required for Umbraco to operate.
     /// </summary>
+    /// <remarks>
+    /// This helps to ensure that everything is registered in the correct order.
+    /// </remarks>
     public class UmbracoApplicationBuilder : IUmbracoApplicationBuilder, IUmbracoEndpointBuilder, IUmbracoApplicationBuilderContext
     {
         private readonly IOptions<UmbracoPipelineOptions> _umbracoPipelineStartupOptions;
 
         public UmbracoApplicationBuilder(IApplicationBuilder appBuilder)
         {
-            ApplicationServices = appBuilder.ApplicationServices;
-            RuntimeState = appBuilder.ApplicationServices.GetRequiredService<IRuntimeState>();
             AppBuilder = appBuilder ?? throw new ArgumentNullException(nameof(appBuilder));
+            ApplicationServices = appBuilder.ApplicationServices;
+            RuntimeState = appBuilder.ApplicationServices.GetRequiredService<IRuntimeState>();            
             _umbracoPipelineStartupOptions = ApplicationServices.GetRequiredService<IOptions<UmbracoPipelineOptions>>();
         }