Add Error Handling for Non-Existent Ticket IDs in Ticket Page

**Description:**
Currently, the `TicketPage` doesn't properly handle the case when the `id` provided does not exist in the system. If the ID doesn't correspond to an existing ticket, the page crashes unexpectedly.

**Expected Behavior:**
- If the `id` does not correspond to any ticket, the user should see a clear error message like "Ticket not found" or be redirected to a custom 404 page.
- The `getTicketById` function should handle invalid cases gracefully by returning a `null` or some indication when the ticket is not found, avoiding any crashes or undefined states.

**Steps to Reproduce:**
1. Attempt to access `TicketPage` with an invalid ticket ID (for eg. try hitting `http://localhost:3000/TicketPage/invalid-ticket-id`).
2. Observe that the app breaks.

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We need to make sure the `getTicketById` function correctly handles failure scenarios, including cases where the API does not return a valid ticket, and shows appropriate feedback on the page.

Start by locating the TicketPage entry point and the getTicketById function, then reproduce the failure with an invalid ID such as /TicketPage/invalid-ticket-id. Trace how a missing or invalid API result reaches the page. Done means the page shows a clear not-found message or custom 404 instead of crashing, and valid ticket pages still work.

Written by the indexing model from the issue text.