BugFix: Do not call unsubscribe agents if it is not a function

[zhen0]

Description

<! -- What is it meant to do? -->

Prevents an error in the terminal that arises from calling unsubscribeAgents when it hasn't been set as a function. This may be a superficial fix that means we are subscribed to agent polling for longer than we need to be. I do not think the overall impact is large but would appreciate @stackoverfloweth's eyes on this to check there is nothing more nefarious.

Linked Issues

<! -- Use a key word (e.g. closes or resolves) to close related issues -->
Closes #1216

Tests and performance

• Changes to any .js files are covered by existing tests or this PR adds new tests (if not please explain why below)
• No new packages are added or package size and performance considerations are discussed below
• PR Title fits our changelog format

Releases/Changelog cuts only

• Completed the QA Checklist in staging

[stackoverfloweth]

@zhen0 seems pretty safe to me 👍