ActionView::Template::Error: undefined method 'created_at' for nil (ActionView::Template::Error)

## Error

```
undefined method 'created_at' for nil (NoMethodError)

def first_submitted_at = oldest_solution.published_iterations.last.created_at
^^^^^^^^^^^
```

## Stack trace

```
app/models/exercise/representation.rb:61 in first_submitted_at
app/views/tracks/community_solutions/show.html.haml:100
action_view/base.rb:263 in public_send
action_view/base.rb:263 in _run
action_view/template.rb:261 in block in ActionView::Template#render
active_support/notifications.rb:206 in block in ActiveSupport::Notifications.instrument
active_support/notifications/instrumenter.rb:58 in instrument
active_support/notifications.rb:206 in ActiveSupport::Notifications.instrument
action_view/template.rb:556 in instrument_render_template
action_view/template.rb:249 in render
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ActionView::Template::Error: undefined method 'created_at' for nil (ActionView::Template::Error)](https://thalamus-ai.sentry.io/issues/95242024/)
- **Culprit**: Tracks::CommunitySolutionsController#show
- **First seen:** Feb 11, 2026