java.lang.NullPointerException: Cannot invoke "java.util.Optional.ifPresent(java.util.function.Consumer)" because the return value of "..."is null

[질문 템플릿]
1. 강의 내용과 관련된 질문인가요? (예)
2. 인프런의 질문 게시판과 자주 하는 질문에 없는 내용인가요? (예)
3. 질문 잘하기 메뉴얼을 읽어보셨나요? (예)
[질문 내용]
스프링 입문 - 코드로 배우는 스프링 부트, 웹 MVC, DB 접근 기술 강의 회원테스트 기술 부분을 학습하고있는데 테스트케이스 오류가 났습니다 어떻게 해결해야할지 몰라 질문글을 적었습니다

아래는 테스트 코드입니다

import org.junit.jupiter.api.Test;
import static org.assertj.core.api.Assertions.*;
import static org.junit.jupiter.api.Assertions.*;

class MemberServiceTest {
    MemberRepository memberRepository = new MemoryMemberRepository();
    MemberService memberService = new MemberService();

    //테스트는 과감하게 한글로 작성 가능
    //테스트할땐 given,when,then 틀 사용

    @Test
    void 회원가입() {
        //given
        Member member = new Member();
        member.setName("hello");
        //when
        Long saveId = memberService.join(member);

        //then
        Member findMember = memberService.findOne(saveId).get();
        org.assertj.core.api.Assertions.assertThat(member.getName()).isEqualTo(findMember.getName());
    }

    @Test
    void findMembers() {
    }

    @Test
    void findOne() {
    }
}

그리고 MemberService코드입니다

package hello.hello_spring.service;

import hello.hello_spring.domain.Member;
import hello.hello_spring.repository.MemberRepository;
import hello.hello_spring.repository.MemoryMemberRepository;

import java.util.List;
import java.util.Optional;

public class MemberService {
    private final MemberRepository memberRepository = new MemoryMemberRepository();

    /*회원가입*/
    public Long join(Member member){
        validateDuplicateMember(member); //중복회원검증
        memberRepository.save(member);
        return member.getId();
    }

    private void validateDuplicateMember(Member member){
        memberRepository.findByName(member.getName())
                .ifPresent(m -> {
                    throw new IllegalArgumentException("이미 존재하는 회원입니다");
                });
    }
    /*전체회원 조회*/
    public List<Member> findMembers() {
        return memberRepository.findAll();
    }

    public Optional<Member> findOne(Long memberId){
        return  memberRepository.findById(memberId);
    }
}

아래는 오류 메시지 입니다

java.lang.NullPointerException: Cannot invoke "java.util.Optional.ifPresent(java.util.function.Consumer)" because the return value of "hello.hello_spring.repository.MemberRepository.findByName(String)" is null

at hello.hello_spring.service.MemberService.validateDuplicateMember(MemberService.java:22)

at hello.hello_spring.service.MemberService.join(MemberService.java:15)

at hello.hello_spring.service.MemberServiceTest.회원가입(MemberServiceTest.java:24)

at java.base/java.lang.reflect.Method.invoke(Method.java:568)

at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

Process finished with exit code -1

어떻게 해결하면 좋을까요?

return store.values().stream()
        .filter(member -> member.getName().equals(name))
        .findAny();

@BeforeEach // 각 테스트 실행 전에 실행됨 public void setUp() {     memberRepository = new MemoryMemberRepository();     memberService = new MemberService(memberRepository); // MemberService에 Repository 주입 

public MemberService(MemberRepository memberRepository) {
    this.memberRepository = memberRepository;
}

private MemberRepository memberRepository = new MemoryMemberRepository();