API Governance Validation in Design Center Fails with "Custom validator exited unexpectedly" (CustomValidatorError) or "Root file model not found"

When running API Governance validation in Anypoint Design Center (the in-browser experience), validation fails. One or both of the following errors may appear.

Error 1 - shown in the UI when clicking the "Validate with governance" / "Validate ruleset" button:

{
  "info": {
    "platform": "Chrome 1xx.x.x.x on Windows 10 64-bit"
  },
  "error": {
    "message": "Custom validator exited unexpectedly",
    "name": "CustomValidatorError"
  }
}

Error 2 - may appear in the browser console (often alongside Error 1), especially on large projects:

{
  "error": {
    "message": "Root file model not found"
  }
}

Additional observations:
- The failures are intermittent and are more likely on complex APIs with many fragments and custom rulesets.
- Governance auto-validation may not trigger automatically for complex projects (for example, when publishing a version to Exchange in Development state); the manual Validate button is then used and produces the errors above.
- Most reproducible on large projects where the governance content is still loading when the Validate button becomes clickable.