Fix fatal error when resolving line discussions with thread auto-collapse feature

The following discussion from !217858 should be addressed:

• @ms.mondrian started a discussion: (+3 comments)

  issue - resolving a file level thread errors out

• This turned out to be not replicable, but did expose a different (basically inert) error:

  I can't replicate this error at all, but do get a console fatal error - that isn't displayed in the UI - on every other (line) collapse.

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The fatal error is displayed in the console, but does not impact either the UI (no banner or notification), and does not block the auto-collapse.