Prevent crash when checking permissions of external reviewer without shares

[emilytoppm]

Check for existence of share before checking share.can_comment to prevent exception for external reviewers without shares

[gasman]

Not sure I follow this logic - if the Share object is the thing that determines whether an external reviewer has access to a page, why is it valid for an external reviewer not to have one?

[emilytoppm]

Not sure I follow this logic - if the Share object is the thing that determines whether an external reviewer has access to a page, why is it valid for an external reviewer not to have one?

I think the idea was a Share was a separate mechanism for sharing a particular draft revision in a time limited fashion (eg the shares panel), but if it was on a particular task state, it should always be reviewable by the external users assigned to that task.