Merge pull request #1036 from microsoft/mk/resolve-null-reference-exception

Add validation for null schema values during V2 response header deserialization

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/V2/OpenApiHeaderDeserializer.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
 using System;
@@ -139,8 +139,12 @@ internal static partial class OpenApiV2Deserializer
                 {
                     OpenApiConstants.Default,
                     new AnyFieldMapParameter<OpenApiHeader>(
-                        p => p.Schema.Default,
-                        (p, v) => p.Schema.Default = v,
+                        p => p.Schema?.Default,
+                        (p, v) => 
+                        {
+                            if(p.Schema == null) return;
+                            p.Schema.Default = v; 
+                        },
                         p => p.Schema)
                 }
             };
@@ -151,8 +155,12 @@ internal static partial class OpenApiV2Deserializer
                 {
                     OpenApiConstants.Enum,
                     new AnyListFieldMapParameter<OpenApiHeader>(
-                        p => p.Schema.Enum,
-                        (p, v) => p.Schema.Enum = v,
+                        p => p.Schema?.Enum,
+                        (p, v) =>
+                        {
+                            if(p.Schema == null) return;
+                            p.Schema.Enum = v; 
+                        },
                         p => p.Schema)
                 },
             };