One of the tx results in a batch loader is null

Error is coming from here resulting in a failed transaction error logged as below. Http response to POST /v1/tx in such case is 500.

Need to investigate the cause and handle the error appropriately.

{
  "severity": "ERROR",
  "@type": "type.googleapis.com/google.devtools.clouderrorreporting.v1beta1.ReportedErrorEvent",
  "time": "2025-05-07T00:50:58.310Z",
  "context": "ErrorHandler",
  "traceId": "83711473af0f6472a932590aa0ab9f4d",
  "spanId": "cc4c078b8ba1ebce",
  "traceFlags": 1,
  "status": 500,
  "message": "Failed to sign and broadcast tx",
  "stack": "InternalServerError: Failed to sign and broadcast tx\n    at assert (/app/apps/api/dist/server.js:103764:9)\n    at BatchSigningClientService.<anonymous> (/app/apps/api/dist/server.js:232972:39)\n    at Generator.next (<anonymous>)\n    at fulfilled (/app/apps/api/dist/server.js:232915:58)\n    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)",
  "data": null
}

Start in apps/api/src/billing/lib/batch-signing-client/batch-signing-client.service.ts at line 89, then trace the POST /v1/tx flow and the batch result that becomes null. Determine the cause and define handling that prevents the failed transaction from producing an inappropriate 500 response; verify the resulting error behavior.