Update src/Microsoft.OpenApi/Models/OpenApiRequestBody.cs

Co-authored-by: Vincent Biret <vibiret@microsoft.com>

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiRequestBody.cs

@@ -137,7 +137,8 @@ public IEnumerable<IOpenApiParameter> ConvertToFormDataParameters(IOpenApiWriter
                             OpenApiSchema s => s, // we already have a copy
                                                   // we have a copy of a reference but don't want to mutate the source schema
                                                   // TODO might need recursive resolution of references here
-                            OpenApiSchemaReference r => (OpenApiSchema?)r.Target?.CreateShallowCopy(),
+                            OpenApiSchemaReference r when r.Target is not null => (OpenApiSchema)r.Target.CreateShallowCopy(),
+                            OpenApiSchemaReference r2 when r.Target is null => throw new InvalidOperationException("Unresolved reference target"),
                             _ => throw new InvalidOperationException("Unexpected schema type")
                         };
                         if (updatedSchema is not null)