[BUG] GraphQL error extensions property undefined if long link is null

[Coteh]

Describe the bug
Browser complains about graphQLError.extensions property being undefined if a null long link is attempted to be converted to string by user short links section. This seems to be because the error response is assumed to contain the extensions field.

To Reproduce

1. Change any short link's long link to null
2. Set user-short-links-section feature to true in app/usecase/feature/static.go
3. Reload frontend homepage

Expected behavior
An error for loading the offending long link should be displayed.

Screenshots

Additional context
Although null long link is indeed an error, a more sensible error should be displayed to the screen if this happens. Additionally, either the extensions property should be defined within the response of this error from backend, or the frontend should check for existence of this property before retrieving the extension code.