Improve the error reported in the log for a NullReferenceException issue

Bắt đầu với ServiceDefinition.cs tại ReferenceTrackSchemas ở dòng 509, sau đó theo Compare ở dòng 321 và SwaggerModeler.cs ở dòng 34. Tái hiện lỗi bằng lệnh OpenApiDiff.dll và các tệp đã được resolve cũ/mới hiển thị trong chi tiết lỗi. Được xem là hoàn tất khi log ghi nhận đủ ngữ cảnh để truy vết vấn đề trong specification bên dưới, thay vì chỉ ghi nhận NullReferenceException.

The following error seems to be caused by spec issues, but it doesn't have enough information to trace the real issues.

Error: appendOadRuntimeErrors: {"type":"Raw","level":"Error","message":"Runtime Exception","time":"2025-08-06T20:28:16.228Z","groupName":"preview","extra":{"new":"https://github.com/stevemunk/azure-rest-api-specs/blob/ec36d0206de5a236b79e079611bcd25df47362cd/specification/maps/data-plane/Microsoft.Maps/Alias/preview/2.0/alias.json","old":"https://github.com/Azure/azure-rest-api-specs/blob/main/specification/maps/data-plane/Microsoft.Maps/Alias/preview/2.0/alias.json","details":"Command failed: dotnet /home/runner/work/azure-rest-api-specs/azure-rest-api-specs/node_modules/\\@azure/oad/dlls/OpenApiDiff.dll -o /tmp/oad-6bw2a3/old-resolved.json -n /tmp/oad-2X5zP2/new-resolved.json\nUnhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.\n   at AutoRest.Swagger.Model.ServiceDefinition.ReferenceTrackSchemas(ServiceDefinition service) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\ServiceDefinition.cs:line 509\n   at AutoRest.Swagger.Model.ServiceDefinition.Compare(ComparisonContext`1 context, ServiceDefinition previousDefinition) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\Model\\ServiceDefinition.cs:line 321\n   at AutoRest.Swagger.SwaggerModeler.Compare(String fileNameOld, String swaggerOld, String fileNameNew, String swaggerNew, Settings settings) in D:\\a\\_work\\1\\s\\openapi-diff\\src\\modeler\\AutoRest.Swagger\\SwaggerModeler.cs:line 34\n   at OpenApiDiff.Program.Main(String[] args) in D:\\a\\_work\\1\\s\\openapi-diff"}}

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