App migrations stuck because migrationGateway.createAppData(...) is throwing NullPointerException

Issue Summary

migrationGateway.createAppData(...) throwing NullPointerException in JCMA when the app migration code executes this method asynchronously after `onStartAppMigration()` has returned. 

public void onStartAppMigration(
    AppCloudMigrationGateway migrationGateway,
    String transferId,
    MigrationDetailsV1 migrationDetails
  ) {
    doMigrationLater(
        () -> gateway.createAppData(transferId)
    );
  }

Resources

• https://developer.atlassian.com/platform/app-migration/app-data/ 
• https://developer.atlassian.com/platform/app-migration/listener/#implement-the-interface

Known Impacted Apps

• Proforma

Steps to Reproduce

1. Install either 1.9.17, 1.10.0 or 1.10.1 of JCMA
2. Install a known impacted app
3. Run migration with impacted app
4. App migration is stuck with no logs indicating app data upload is successful or a NullPointerException in migrationGateway.createAppData() is found in the logs

Expected Results

App data is uploaded successfully.

Actual Results

App data fails to upload and app migration is stuck

Workaround

Downgrade to JCMA 1.9.16