Is this correct usage of builder pattern in dtos?

Assume i have PostDTO that has this fields:

private Long id;
private ShortInfoUserDTO author;
private CategoryDTO category;

And i have CategoryDTO inside of it that also has Builder, and it has fields:

private Long id;
private String name;

And ShortInfoUserDTO has some fields (doesn't matter what fields, but it also has Builder)

And My builder pattern looks like this ( both CategoryDTO and

public static Builder builder() {
    return new PostDTO.Builder();
}
public static class Builder {
    PostDTO instance = new PostDTO();        

    public Builder id(Long id) {
        instance.id = id;
        return this;
    }

    public Builder author(User user) {
        ShortInfoUserDTO author = ShortInfoUserDTO.builder()
                                                  .id(user.getId())
                                            .username(user.getUsername())
                                            .build();
        instance.author = author;
        return this;
    }



    public Builder category(Category category) {
          CategoryDTO categoryDto = CategoryDTO
                                      .builder()
                                      .id(category.getId())
                                      .name(category.getName())
                                      .build();           

          instance.category = categoryDto;            
          return this;
    }

    public PostDTO build() {
        return instance;
    }
}

Is generally the idea of building author() and category() this way good? like is this proper architecture? One of the reason im asking is because i'm kinda worried about nullpointerexceptions