Illuminate\Database\Eloquent\Relations\BelongsToMany::save(): Argument #1 ($model) must be of type Illuminate\Database\Eloquent\Model, null given

Question

I'm trying to develop a simple freelancing platform with laravel breeze. When i tried to implement a massaging feature between freelancer and admin-(jobs are posted by admin not clients) im getting this error.

Illuminate\Database\Eloquent\Relations\BelongsToMany::save(): Argument #1 ($model) must be of type Illuminate\Database\Eloquent\Model, null given, called in C:\Users\user\freelance_graphicwebefef\app\Http\Controllers\ThreadController.php on line 23

Thread Model

class Thread extends Model
{
    use HasFactory;
    use SoftDeletes;

    public function Users(){
        return $this->belongsToMany('App\Models\User');
    }

    public function Messages(){
        return $this->hasMany('App\Models\Message','thread_id');
    }

    public function Job(){
        return $this->belongsTo('App\Models\Job');
    }
}

User Model

namespace App\Models;

use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Laravel\Sanctum\HasApiTokens;
use Laratrust\Traits\LaratrustUserTrait;

class User extends Authenticatable
{
    use LaratrustUserTrait;
    use HasApiTokens, HasFactory, Notifiable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array<int, string>
     */
    protected $fillable = [
        'name',
        'email',
        'password',
    ];

    /**
     * The attributes that should be hidden for serialization.
     *
     * @var array<int, string>
     */
    protected $hidden = [
        'password',
        'remember_token',
    ];

    /**
     * The attributes that should be cast.
     *
     * @var array<string, string>
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];

    public function role(){
        return $this->belongsTo('App\Models\Role');
    }

    public function hasRoleCustom($Role){
        return null !== $this->role()->where('name',$Role)->first();
    }

    public function jobs(){
        return $this->hasMany('App\Models\Job','user_id');
    }

    public function Favorites(){
        return $this->hasMany('App\Models\Favorites');
    }

    public function Threads(){
        return $this->belongsToMany('App\Models\Thread');
    }

    public function Message(){
        return $this->hasMany('App\Models\Message','user_id');
    }
}

ThreadController

class ThreadController extends Controller
{
    public function createThread(Request $request){
        $this->validate($request,[
            'job_id'=>'required|integer'
        ]);
        $Thread=new Thread;
        $Thread->job_id=$request->input('job_id');
        $Thread->save();

        $Freelancer=Auth::user();
        $Admin=$Thread->Job->user;
        $Thread->Users()->save($Freelancer);//**line 22**
        $Thread->Users()->save($Admin);//**line 23**

        return redirect('/threads/'.$Thread->id);

    }

    public function createMessage(Request $request,$id){
        $this->validate($request,[
            'message'=>'required|string',
        ]);
        $Message=new Message();
        $Message->message=$request->input('message');
        $Message->thread_id=$id;

        $User=Auth::user();
        $User->Message()->save($Message);

        return redirect('/threads/'.$id)->with('success','Message Saved');

    }

    public function showThread($id){
        $Thread=Thread::find($id);
        return view('threads.show')->with('Thread',$Thread);
    }
}

PHPStorm gives me this message on line 22

$Thread->Users()->save($Freelancer);

Expected parameter of type '\Illuminate\Database\Eloquent\Model', '\Illuminate\Contracts\Auth\Authenticatable' provided

Thread Table and thread_users table show some records.