=== GET /books/null-demo/legacy/0 (expect 500, NPE) ===
HTTP/1.1 500 
Content-Type: application/json
Transfer-Encoding: chunked
Date: Thu, 08 Oct 2026 05:30:22 GMT
Connection: close

{"timestamp":"2026-10-08T05:30:22.108Z","status":500,"error":"Internal Server Error","path":"/books/null-demo/legacy/0"}
--- server log (stack trace) ---
java.lang.NullPointerException: titleForId(...) must not be null
	at com.ankurm.kotlin.BookController.legacyNullDemo(BookController.kt:47) ~[!/:1.0.0]