jenssegers / laravel-mongodb throws Attempt to read property "users" on null

I am using ' jenssegers /laravel-mongodb-core' package for Laravel+MongoDB connectivity. I am facing issues in the implementation of 'Many to Many' relationship in a default Laravel project but I only receive the following error:

"Attempt to read property "users" on null"

I even tried it with custom foreign keys but I did not get desired results and always get error message. I also know that this package not supports pivot tables for Many to Many relationships.

Please guide that how fix this issue

I have two models (user, role) and below is the code of my Models:

USER MODEL

namespace App\Models;

use Jenssegers\Mongodb\Eloquent\Model;


class user extends Model
{
   // protected $connection = 'mongodb';
    protected $collection = 'users';


    public function roles(){
        return $this->belongsToMany(role::class,null,'role_id_custom');
    }
}

ROLE MODEL

<?php

namespace App\Models;

#use Illuminate\Database\Eloquent\Factories\HasFactory;
#use Illuminate\Database\Eloquent\Model;

use Jenssegers\Mongodb\Eloquent\Model;

class role extends Model
{

   // protected $connection = 'mongodb';
    protected $collection = 'roles';

    public function users(){
        return $this->belongsToMany(user::class,null);
    }
}

HOME Controller

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use App\Models\user;
use App\Models\role;

class home extends Controller
{

    public function index()
    {
        $role = user::find(1)->roles;
        return $role;

       // $user = role::find(1)->users;
        //return $user;
    }
    //
}

SCHEMA of MONGODB

USERS
_id:
name:
role_id_custom: (This is a foreign key)

ROLES
_id:
role:
role_id_custom

Please guide where is the issue? Is this is a package issue?

Many Thanks