Testing Java Service

I have a problem with testing java service My service:

@Service
public class UserService {
    private final UserRepository userRepository;
    private final BCryptPasswordEncoder passwordEncoder;

    public UserService(UserRepository userRepository, BCryptPasswordEncoder passwordEncoder) {
        this.userRepository = userRepository;
        this.passwordEncoder = passwordEncoder;
    }

    public void saveUser(User user) {
        user.setPassword(passwordEncoder.encode(user.getPassword()));
        userRepository.save(user);
    }

    public User findByEmail(String email) {
        return userRepository.findByEmail(email);
    }

    public User findById(Integer id) {

        return userRepository.findById(id).orElse(null);
    }

    public void save(User user) {
        userRepository.save(user);
    }

And my test class:

public class UserServiceTest {
    private UserService userService;

    @Test
    public void saveUser() {

        //given
        User user = new User(1, "Adam", "Nowak", "j@gmail.com", "abcd");
        //when
        userService.saveUser(user);
        User userToFind = userService.findById(1);
        //then
        assertNotNull(userToFind);
    }

When try to run test i have an error:

java.lang.NullPointerException
    at pl.coderslab.projectfilmweb.service.UserServiceTest.saveUser(UserServiceTest.java:18)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

Someone can help with resolving the problem