SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'cart_id' cannot be null

I want to add the product to the cart but it gives an error

class Cart extends Model
{
    protected $guarded = [];


    public function courses()
    {
        return $this->belongsToMany(Course::class, 'cart_course' , 'cart_id' , 'course_id');
    }

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

}

        Schema::create('carts', function (Blueprint $table) {
            $table->id();
            $table->foreignId('user_id')->constrained('users')->cascadeOnDelete();
            $table->integer('total_amount');
            // $table->boolean('is_used_discount')->default(0);
            $table->integer('price');
            // $table->bigInteger('discount_percent');
            $table->bigInteger('total_discount');
            $table->dateTime('expired_at');
            $table->timestamps();
        });

        Schema::create('cart_course', function (Blueprint $table) {
            $table->foreignId('cart_id')->constrained('carts')->cascadeOnDelete();
            $table->foreignId('course_id')->constrained('courses')->cascadeOnDelete();
        });

        <form action="{{route('single-add-to-cart')}}" method="post">
            @csrf
            <input type="hidden" name="course_id" value="{{$course->id}}" id="">
            <button type="submit" class="btn btn-info w-full">Add To Cart</button>
        </form>

    Route::get('/cart/{course}' , 'CartController@addToCart')->name('add-to-cart');
    Route::post('/cart' , 'CartController@singeCart')->name('single-add-to-cart');

    public function addToCart(Course $course ,Request $request)
    {
        $course = Course::where('id' , $request->course_id)->first();

        if(empty($this->course_id)){
            Alert::error('خطا' , 'هیچ محصولی در سبد خرید شما وجود ندارد' , 'error');
        }

        $activeCart = Cart::where('user_id' , auth()->id())->first();

        if($activeCart){
            $activeCart->total_amount = $course->price + $activeCart->total_amount;
            $activeCart->save();
        }

        $cart = new Cart();
        $cart->user_id = auth()->user()->id;
        $cart->price = $course->price;
        $cart->total_discount = 3000;
        $cart->total_amount =  $cart->price + $course->price;

        $cart->courses()->attach($request->course_id);

        $cart->save();

        Alert::success('موفقیت آمیز' , 'محصول با موفقیت به سبد خرید شما اضافه شد' , 'success');

    }