NullPointerException thrown during junit test without @SpringBootTest annotation, but not with the annotation

When I remove the @SpringBootTest annotation, I get a NullPointerException during this test:

@SpringBootTest
@RunWith(MockitoJUnitRunner.class)
public class ExceptionInterceptorTests {

      private AysUserProvisException aysUserProvisException =
          new AysUserProvisException("Failed","True", "Failed to create user. User already exists.", null);

      @InjectMocks @Spy ExceptionInterceptor exceptionInterceptorSpy;
      
      @Test
      void testAysUserProvisException_generateCorrectResponseSchema() {
        ResponseEntity<Object> response = exceptionInterceptorSpy.handleAysUserProvisException(aysUserProvisException);
        
        AysUserProvisResponse exceptionResponse =
            new AysUserProvisResponse(
                "Failed", "True", "Failed to create user. User already exists.", null);
        ResponseEntity<Object> expected =
            new ResponseEntity<>(exceptionResponse, HttpStatus.INTERNAL_SERVER_ERROR);

        assertEquals(response.getBody(), expected.getBody());
      }

It is thrown when attempting to execute this method:

    @ControllerAdvice
    public class ExceptionInterceptor extends ResponseEntityExceptionHandler {
        
        @ExceptionHandler(AysUserProvisException.class)
          public final ResponseEntity<Object> handleAysUserProvisException(AysUserProvisException ex) {
            AysUserProvisResponse exceptionResponse =
                new AysUserProvisResponse(
                    ex.getStatus(), ex.getIsErrorOccurred(), ex.getMessage(), ex.getError());
            return new ResponseEntity<>(exceptionResponse, HttpStatus.INTERNAL_SERVER_ERROR);
        }

Here is the AysUserProvisResponse class:

public class AysUserProvisResponse {
    
    private String status;
    private String isErrorOccurred;
    private String message;
    private Error  error = new Error();
    
    
    public AysUserProvisResponse() {
        super();
    }

    public AysUserProvisResponse(String status, String isErrorOccurred, String message, Error error) {
        super();
        this.status = status;
        this.isErrorOccurred = isErrorOccurred;
        this.message = message;
        this.error = error;
    }

How does the @SpringBootTest annotation avoid this exception? Why is it necessary?