fix handling of global debug message

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Global debug message causes editor runtime error.
This PR try to fix the issue.

Steps to reproduce:

1. Import web socket node example and deploy it,
2. Delete the imported flow,
3. Re-Deploy,
4. (3) causes Error: stopping node: Close timed out,
5. (4) results in editor runtime error TypeError: Cannot read properties of undefined.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 68.33% when pulling 67f4553 on node-red-hitachi:fix-debug-message-handling into b2bb656 on node-red:master.