@ExceptionHandler not working as it was supposed to

Question

I am watching a tutorial about RESTful web services and I'm creating a basic API. At some point it introduced me to exception handling so to test it I made this class:

@ControllerAdvice
public class AppExceptionsHandler extends ResponseEntityExceptionHandler {

    @ExceptionHandler(value = {Exception.class})
    public ResponseEntity<CustomErrorMessage> handleAnyException(Exception ex, WebRequest request) {

        String customErrorMessageDescription = ex.getLocalizedMessage();
        if (customErrorMessageDescription == null)
            customErrorMessageDescription = ex.toString();

        CustomErrorMessage customErrorMessage = new CustomErrorMessage(new Date(), customErrorMessageDescription);

        return new ResponseEntity<CustomErrorMessage>(customErrorMessage, new HttpHeaders(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

And a simple test I was shown was to catch a NullPointerException with the intended error inside the getUser method which is implemented in the UserController class:

@GetMapping(path = "/{userId}", produces = { MediaType.APPLICATION_JSON_VALUE, MediaType.APPLICATION_XML_VALUE })
    public ResponseEntity<UserRest> getUser(@PathVariable String userId) {

        String firstName = null;

        int length = firstName.length();

        if (users.containsKey(userId)) {
            return new ResponseEntity<UserRest>(users.get(userId), HttpStatus.OK);
        } else {
            return new ResponseEntity<UserRest>(HttpStatus.NO_CONTENT);
        }
    }

In the tutorial it shows that when running the code and sending a GET request through Postman you receive the corresponding body and information you requested in the handler with no backtrace in the console, but instead I get a NullPointerException error with backtrace in my console and in Postman I get the default structure of the data which is like this:

{
    "timestamp": 1699010452939,
    "status": 500,
    "error": "Internal Server Error",
    "path": "/users/bb3bd1f5-ab81-40f0-a025-d009e8e30ada"
}

And the backtrace I get is this:

2023-11-03T13:20:52.933+02:00 ERROR 21760 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: java.lang.NullPointerException: Cannot invoke "String.length()" because "firstName" is null] with root cause

java.lang.NullPointerException: Cannot invoke "String.length()" because "firstName" is null
    at com.appsdeveloperblog.app.ws.mobileappws.ui.controller.UserController.getUser(UserController.java:34) ~[classes/:na]
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:580) ~[na:na]
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:253) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:181) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:118) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:917) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:829) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1089) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.15.jar:6.0]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.1.0-RC2.jar:6.1.0-RC2]
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.15.jar:6.0]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) ~[tomcat-embed-websocket-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.springframework.web.filter.ServerHttpObservationFilter.doFilterInternal(ServerHttpObservationFilter.java:109) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116) ~[spring-web-6.1.0-RC2.jar:6.1.0-RC2]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:167) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:340) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:391) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:896) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1744) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-10.1.15.jar:10.1.15]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]

In case someone wants to see the CustomErrorMessage class:

public class CustomErrorMessage {
    private Date timestamp;
    private String message;

    public CustomErrorMessage() {
    }

    public CustomErrorMessage(Date timestamp, String message) {
        this.timestamp = timestamp;
        this.message = message;
    }

    public Date getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(Date timestamp) {
        this.timestamp = timestamp;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

}

I also placed a breakpoint inside the handler but it never gets used from the application. Do I have to add something else to invoke it? I also changed the Exception.class in the @ExceptionHandler annotation to NullPointerException.class in case it would catch the error but nothing changed. I'm a beginner in Spring Boot so I ran out of ideas pretty quickly. It is really frustrating because I did exactly what it showed in the tutorial but mine isn't working.

EDIT

I made it work. It seems that the package that included my handler was not initially scanned by Spring so it couldn't use the annotations I had. With the @ComponentScan("Name of the package") annotation in the main class of the application everything worked correctly.