How to authorize in MockMVC Unit Tests

i am trying to write code to my @Controller secured endpoint, here is the most essential parts of that method:

@GetMapping("/{username}")
    public String getUser(@PathVariable("username") String username, @AuthenticationPrincipal SecurityUser u, Model model){
        User user = userService.readByUsername(username);
        List<Post> posts = postService.getPostsByUser_Username(username);


        boolean ifImageIsPresent = false;
        boolean ifFriend = false;
        boolean isAccount = false;

        Friend f = friendService.getFriendByReceiverUsername(username, u.getUsername());

        if(u.getImages().size() > 0){
            ifImageIsPresent = true;
        }

        if(f != null){
            ifFriend = true;
        }

        if(u.getReceivedRequests().contains(f)){
            ifFriend = true;
        }
        if(u.getUsername().equals(username)){
            isAccount = true;
        }

        model.addAttribute("ifFriend", ifFriend);
        model.addAttribute("isAccount", isAccount);
        model.addAttribute("imageIsPresent", ifImageIsPresent);
        model.addAttribute("posts", posts);

        model.addAttribute("user", user);
        model.addAttribute("image", user.getImages());

        model.addAttribute("size", user.getImages().size());
        return "profile-page";
    }

And here is my test method:

@Test
//    @DisplayName("Test requesting specific user information by user id in URL")
    public void getUserByIdTest() throws Exception {
        User user = new User();
        //Fields by which I will check correctness
        user.setFirstName("FirstTest");
        user.setLastName("LastTest");
        user.setPassword("q");
        user.setEmail("[email protected]");
        user.setUsername("q");
        user.setId(444);

        given(userService.readByUsername(user.getUsername())).willReturn(user);

        mockMvc.perform(MockMvcRequestBuilders.get("/users/q"))
                .andExpect(view().name("profile-page"))
                .andExpect(status().isUnauthorized())
                .andExpect(model().attributeExists("user"))
                .andExpect(model().attribute("user", user));
    }

And in respose i get that i`m not authorize, so 401 error.

I tried adding annotation

@WithMockUser(username = "q", password = "q", roles = "USER")

But then in logs i get this error:

Caused by: java.lang.NullPointerException: Cannot invoke "ua.socialnetwork.security.SecurityUser.getUsername()" because "u" is null
    at ua.socialnetwork.controller.UserController.getUser(UserController.java:136)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:207)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:152)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1080)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:973)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1010)
    ... 137 more

Also, i have tried adding

@Import(SecurityConfig.class)

But it did not help.

I think the problem is in method, because i am using

@AuthenticationPrincipal annotation

But i havent found any solutions how to fix it

I need to somehow login in MockMVC, so i can test my code:D Or think of other methods, but i dont know which ones and actually how, because kind of new to tests