"retrieveUser returned null - a violation of the interface contract" in springboot security when authentication

Question

I am new to springboot security and i am trying write the signup function.My approach is to save the user and then pass the data to the autheicationmanager,but the went in here and it return null and the above error occur. token service:

public String signup(JpaUser jpaUser) {
        System.out.println(jpaUserrepository.findByUsername(jpaUser.getUsername()));
       if(jpaUserrepository.findByUsername(jpaUser.getUsername()).isPresent())
        {return "repeated username";}
        JpaUser saveuser=jpaUserrepository.save(new JpaUser(jpaUser.getUsername(),
                passwordEncoder.encode(jpaUser.getPassword()),jpaUser.getEmail(),jpaUser.getRoles()));
        Authentication authentication= authenticationManager.authenticate(new UsernamePasswordAuthenticationToken(
                saveuser.getUsername(),saveuser.getPassword()));
        System.out.println(authentication);
        String token=generateToken(authentication);
        System.out.println(token);
       return token;

    }

securityconfig:

@Bean
public AuthenticationManager authenticationManager(AuthenticationConfiguration authConfig)  {
    try{System.out.println(authConfig.getAuthenticationManager());
    return authConfig.getAuthenticationManager();}catch(Exception exception){
        System.out.println(exception);
        return null;
    }
}

generate token method:

 public String generateToken (Authentication authentication){
        Instant now=Instant.now();
        String scope=authentication.getAuthorities().stream() //Stream<capture of ? extends GrantedAuthority >
        .map(GrantedAuthority::getAuthority)
                .collect(Collectors.joining(" "));
        JwtClaimsSet claims = JwtClaimsSet.builder()
                .issuer(authentication.getName())
                .issuedAt(now)
                .expiresAt(now.plus(1, ChronoUnit.HOURS))
                .claim("scope",scope).build();
                return this.encoder.encode(JwtEncoderParameters.from((claims))).getTokenValue();
    }