Fix undefined local variable error

[lfdebrux]

What problem does this pull request solve?

Make sure that we pass current_form to the type_of_answer template in the (very rare) case where the answer type input is invalid.

This should resolve https://govuk-forms.sentry.io/issues/7557924147/.

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• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[github-actions]

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