Sub Query of User detail not working

I have News Articles created by users. The page gets the list of all the News with the IDs of the authors. The problem is, the details requested by the sub query for the authors returns null.

GraphQL Query

newsByPage(
          first: $first
          page: $page
        ){
          data{
            id
            title
            body
            user_id{
              id
              firstname
              lastname
              gender
            }
            is_active
            created_at
          }

          paginatorInfo{
            firstItem
            total
          }
        }

Type Schema

type NewsList {
    id: ID!
    is_active: Boolean!
    title: String!
    body: String!
    user_id: AppUsers!
    created_at: DateTime!
    updated_at: DateTime
}

extend type Query {
    newsByPage: [NewsList!]! @paginate(defaultCount: 10) @orderBy(column: "created_at", direction: DESC)
}

NewsList Model

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;

class NewsList extends Model
{
    protected $fillable = [
            'id',
            'is_active',
            'title',
            'body',
            'user_id'
    ];

    public function user_id(): BelongsTo
    {
        return $this->belongsTo(AppUsers::class);
    }
}

AppUsers Model

use Illuminate\Database\Eloquent\Model;
class AppUsers extends Model
{
    public $incrementing = false;
    protected $keyType = 'string';

    protected $fillable = [
        'id',
        'is_active',
        'firstname',
        'lastname',
        'gender',
        'city',
        'country',
        'phone',
        'email',
        'leadership',
        'user_group',
        'user_zone',
        'level'
    ];

    public function id()
    {
        return $this->hasMany(
            UserAuths::class, UserSettings::class, Content1Label::class, NewsList::class
        );
    }
}

Migration

        Schema::create('news_list', function (Blueprint $table) {
            $table->uuid('id')->primary();
            $table->boolean('is_active');
            $table->string('title');
            $table->mediumText('body');
            $table->uuid('user_id');
            $table->timestamp('created_at')->useCurrent();
            $table->timestamp('updated_at')->useCurrent();
        });

// just found out the below was missing (relationship), which has been added to the database
// and migration file too
        Schema::table('news_list', function (Blueprint $table) {
            $table->foreign('user_id')->references('id')->on('app_users')->onDelete('cascade')->onUpdate('cascade');
        });

Error

production.ERROR: Cannot return null for non-nullable field "AppUsers.id". 
{"exception":"[object] (GraphQL\Error\InvariantViolation(code: 0): 
Cannot return null for non-nullable field \"AppUsers.id\". at C:\SmartApp\vendor\webonyx\graphql-php\src\Executor\ReferenceExecutor.php:748)

Help please

What could be wrong with my set-up? I appreciate any help.