fix: nullref on walking optional reply property refs

src/ByteBard.AsyncAPI.Readers/AsyncApiJsonDocumentReader.cs

@@ -280,7 +280,7 @@ private IAsyncApiSerializable ResolveExternalReference(AsyncApiDiagnostic diagno
                 }
                 else
                 {
-                    stream = loader.Load(new Uri(reference.Reference.ExternalResource, UriKind.RelativeOrAbsolute));
+                    stream = loader.Load(this.settings.BaseUri, new Uri(reference.Reference.ExternalResource, UriKind.RelativeOrAbsolute));
                     this.context.Workspace.RegisterComponent(reference.Reference.ExternalResource, stream);
                 }
 
@@ -310,7 +310,7 @@ private async Task<IAsyncApiSerializable> ResolveExternalReferenceAsync(AsyncApi
                 }
                 else
                 {
-                    stream = await loader.LoadAsync(new Uri(reference.Reference.ExternalResource, UriKind.RelativeOrAbsolute));
+                    stream = await loader.LoadAsync(this.settings.BaseUri, new Uri(reference.Reference.ExternalResource, UriKind.RelativeOrAbsolute));
                     this.context.Workspace.RegisterComponent(reference.Reference.ExternalResource, stream);
                 }
 

src/ByteBard.AsyncAPI.Readers/AsyncApiReaderSettings.cs

@@ -70,6 +70,11 @@ public ICollection<IBindingParser<IBinding>>
         /// </summary>
         public bool LeaveStreamOpen { get; set; }
 
+        /// <summary>
+        /// Uri where relative references should be resolved from when using the External Reference Loader.
+        /// </summary>
+        public Uri? BaseUri { get; set; }
+
         /// <summary>
         /// External reference reader implementation provided by users for reading external resources.
         /// </summary>

src/ByteBard.AsyncAPI.Readers/Interface/IStreamLoader.cs

@@ -6,8 +6,8 @@ namespace ByteBard.AsyncAPI.Readers.Interface
 
     public interface IStreamLoader
     {
-        Task<Stream> LoadAsync(Uri uri);
+        Task<Stream> LoadAsync(Uri baseUri, Uri uri);
 
-        Stream Load(Uri uri);
+        Stream Load(Uri baseUri, Uri uri);
     }
 }

src/ByteBard.AsyncAPI.Readers/Services/DefaultStreamLoader.cs

@@ -11,40 +11,54 @@ internal class DefaultStreamLoader : IStreamLoader
     {
         private static readonly HttpClient HttpClient = new HttpClient();
 
-        public Stream Load(Uri uri)
+        public Stream Load(Uri baseUri, Uri uri)
         {
             try
             {
-                switch (uri.Scheme)
+                if (uri.IsAbsoluteUri)
                 {
-                    case "file":
-                        return File.OpenRead(uri.AbsolutePath);
-                    case "http":
-                    case "https":
-                        return HttpClient.GetStreamAsync(uri).GetAwaiter().GetResult();
-                    default:
-                        throw new ArgumentException("Unsupported scheme");
+                    switch (uri.Scheme.ToLowerInvariant())
+                    {
+                        case "file":
+                            return File.OpenRead(uri.LocalPath);
+                        case "http":
+                        case "https":
+                            return HttpClient.GetStreamAsync(uri).GetAwaiter().GetResult();
+                        default:
+                            throw new ArgumentException("Unsupported scheme");
+                    }
+                }
+                else
+                {
+                    return File.OpenRead(new Uri(baseUri, uri).LocalPath);
                 }
             }
             catch (Exception ex)
             {
-                throw new AsyncApiReaderException($"Something went wrong trying to fetch '{uri.OriginalString}. {ex.Message}'", ex);
+                throw new AsyncApiReaderException($"Something went wrong trying to fetch '{uri.OriginalString}'. {ex.Message}", ex);
             }
         }
 
-        public async Task<Stream> LoadAsync(Uri uri)
+        public async Task<Stream> LoadAsync(Uri baseUri, Uri uri)
         {
             try
             {
-                switch (uri.Scheme)
+                if (uri.IsAbsoluteUri)
+                {
+                    switch (uri.Scheme.ToLowerInvariant())
+                    {
+                        case "file":
+                            return File.OpenRead(uri.LocalPath);
+                        case "http":
+                        case "https":
+                            return await HttpClient.GetStreamAsync(uri);
+                        default:
+                            throw new ArgumentException("Unsupported scheme");
+                    }
+                }
+                else
                 {
-                    case "file":
-                        return File.OpenRead(uri.AbsolutePath);
-                    case "http":
-                    case "https":
-                        return await HttpClient.GetStreamAsync(uri);
-                    default:
-                        throw new ArgumentException("Unsupported scheme");
+                    return File.OpenRead(new Uri(baseUri, uri).LocalPath);
                 }
             }
             catch (Exception ex)
@@ -53,4 +67,4 @@ public async Task<Stream> LoadAsync(Uri uri)
             }
         }
     }
-}
+}

src/ByteBard.AsyncAPI/Services/AsyncApiWalker.cs

@@ -587,8 +587,15 @@ private void Walk(AsyncApiOperationReply reply)
 
             this.visitor.Visit(reply);
 
-            this.Walk(reply.Address);
-            this.Walk(reply.Channel as IAsyncApiReferenceable);
+            if (reply.Address != null)
+            {
+                this.Walk(reply.Address);
+            }
+
+            if (reply.Channel != null)
+            {
+                this.Walk(reply.Channel as IAsyncApiReferenceable);
+            }
 
             foreach (var message in reply.Messages)
             {
@@ -1211,4 +1218,4 @@ public void Walk(IAsyncApiElement element)
             }
         }
     }
-}
+}

test/ByteBard.AsyncAPI.Tests/Models/AsyncApiReference_Should.cs

@@ -525,7 +525,7 @@ public MockStringLoader(string input)
 
         private readonly string input;
 
-        public Stream Load(Uri uri)
+        public Stream Load(Uri baseUri, Uri uri)
         {
             var stream = new MemoryStream();
             var writer = new StreamWriter(stream);
@@ -535,9 +535,9 @@ public Stream Load(Uri uri)
             return stream;
         }
 
-        public Task<Stream> LoadAsync(Uri uri)
+        public Task<Stream> LoadAsync(Uri baseUri, Uri uri)
         {
-            return Task.FromResult(this.Load(uri));
+            return Task.FromResult(this.Load(baseUri, uri));
         }
     }
 
@@ -564,7 +564,7 @@ public class MockJsonSchemaLoader : IStreamLoader
                 description: Light intensity measured in lumens.
             """;
 
-        public Stream Load(Uri uri)
+        public Stream Load(Uri baseUri, Uri uri)
         {
             var stream = new MemoryStream();
             var writer = new StreamWriter(stream);
@@ -581,9 +581,9 @@ public Stream Load(Uri uri)
             return stream;
         }
 
-        public Task<Stream> LoadAsync(Uri uri)
+        public Task<Stream> LoadAsync(Uri baseUri, Uri uri)
         {
-            return Task.FromResult(this.Load(uri));
+            return Task.FromResult(this.Load(baseUri, uri));
         }
     }
 }