خطأ في علاقة Eloquent Laravel 8

أتلقى الخطأ التالي في مشروعي الذي أستعمل فيه الإصدار الثامن:

Attempt to read property "created_by" on null

هذا ما يوجد في ملف العرض:

<h6><strong>Created By:</strong> {{ $purchase->user->created_by }}</h6>
<h6><strong>Updated By:</strong> {{ $purchase->user->updated_by }}</h6>

حيث النموذج User:

<?php

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;

class User extends Authenticatable
{
    use HasFactory, Notifiable;

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

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password',
        'remember_token',
    ];

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

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

    public function scopeAuthors($query){
      return $query->where('role_id',2);
    }

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

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

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

و النموذج Purchase:

<?php

namespace App\Models;

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

class Purchase extends Model
{
    use HasFactory;

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

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

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

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

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

ملف التهجير الخاص بالمستخدمين:

public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->id();
            $table->integer('role_id')->default(3);
            $table->string('name');
            $table->string('username')->unique();
            $table->string('email')->unique();
            $table->timestamp('email_verified_at')->nullable();
            $table->string('password');
            $table->string('image')->default('default.png');
            $table->rememberToken();
            $table->timestamps();
        });
    }

ملف التهجير الخاص بالمشتريات:

public function up()
    {
        Schema::create('purchases', function (Blueprint $table) {
            $table->id();
            $table->string('purchase_no');
            $table->date('date');
            $table->double('buying_qty');
            $table->double('unit_price');
            $table->double('buying_price');
            $table->boolean('status')->default('0');
            $table->integer('supplier_id');
            $table->integer('category_id');
            $table->integer('product_id');
            $table->integer('created_by');
            $table->integer('updated_by')->nullable();
            $table->text('description')->nullable();
            $table->timestamps();
        });
    }

ماهو حل المشكلة.