Guard against null references

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/OpenApiYamlReader.cs

@@ -48,7 +48,7 @@ public ReadResult Read(MemoryStream input,
             // Parse the YAML text in the TextReader into a sequence of JsonNodes
             try
             {
-                using var stream = new StreamReader(input, default, true, -1, settings.LeaveStreamOpen);
+                using var stream = new StreamReader(input, default, true, -1, settings?.LeaveStreamOpen);
                 jsonNode = LoadJsonNodesFromYamlDocument(stream);
             }
             catch (JsonException ex)

src/Microsoft.OpenApi/Reader/OpenApiModelFactory.cs

@@ -210,7 +210,7 @@ private static async Task<ReadResult> InternalLoadAsync(Stream input, string for
             var reader = OpenApiReaderRegistry.GetReader(format);
             var readResult = await reader.ReadAsync(input, settings, cancellationToken);
 
-            if (settings.LoadExternalRefs)
+            if (settings is not null && settings.LoadExternalRefs)
             {
                 var diagnosticExternalRefs = await LoadExternalRefsAsync(readResult.OpenApiDocument, cancellationToken, settings, format);
                 // Merge diagnostics of external reference
@@ -239,7 +239,7 @@ private static async Task<OpenApiDiagnostic> LoadExternalRefsAsync(OpenApiDocume
         private static ReadResult InternalLoad(MemoryStream input, string format, OpenApiReaderSettings settings = null)
         {
             Utils.CheckArgumentNull(format, nameof(format));
-            if (settings.LoadExternalRefs)
+            if (settings is not null && settings.LoadExternalRefs)
             {
                 throw new InvalidOperationException("Loading external references are not supported when using synchronous methods.");
             }