how to solve (Trying to get property of non-object) error, but the same function works well in another function

what is wrong with this controller, I always get this error when update order " Trying to get property of non-object " ,(notice that I'm using the same function for store and it works fine.

I have 4 tables in my database (user_type_id , users , products and orders, orders table contains 4 columns (id , product_id , user_id and qty), user_id and product_id = foreign keys of users and products tables.

also I want something I do not know how to write it correctly, I want when a customer delete an order and it is only one order (quantity = 1), to be deleted permanently and also if he typed the same quantity, to be deleted permanently also because when I tested this function I found that it keeps deleting till -1 -2 -3 etc..

and I want if the product quantity is 0 to hide the product for all users except the admins and the seller who sold it or at least to hide the Add to Cart button for all or replace it with dead button with out of stock instead of Add to Cart and if a user already ordered some of this product he cannot update quantity but can cancel it then it will be added to products again..

and I hope if someone told me how to use the commented spans because I do not know how to do that also :D thanks

    namespace App\Http\Controllers;
    use Illuminate\Http\Request;
    use App\Http\Requests;
    use App\Order;
    use App\Product;
    use Auth;
    use App\User;
    use App\Details;
    use Validator;
    
    class OrderController extends Controller
    {
    
        public function __construct()
        {
            $this->middleware('auth');
        }
    
        public function myorders()
        {
            $orders = Order::where('user_id','=',Auth::user()->id)->orderBy('id','desc')->get();
            return view('order.myorders',compact('products','orders','users'));
        }   
    
    
    
    
    
    
      public function buy(Request $id)
        {
            $product = Product::find($id);
            return view('product.index',compact('product','order','user'));
        }
    
    public function store(Request $request, $id)
        
    
            {
            $validator = Validator::make($request->all(), [
                'qty' => 'required|max:255',
                
            ]);
    
            if ($validator->fails()) {
                return redirect('')
                            ->withErrors($validator)
                            ->withInput();
            }
    
            $product = Product::find($id);
            $order = new Order();
            $order->user_id = Auth::user()->id;
            $order->product_id = $id;
            $order->qty = $request['qty'];
            $product->inv = $product->inv - $order->qty;
            $order->save();
            $product->save();
            return redirect('/');
        }
     
        
    public function edit(Request $id)
        {
            $product = Product::find($id);
            return view('order.myorders',compact('product','order','user'));
        }
    
    
    public function update(Request $request, $id)
        
    
            {
            $validator = Validator::make($request->all(), [
                'qty' => 'required|max:255',
                
            ]);
    
            if ($validator->fails()) {
                return redirect('')
                            ->withErrors($validator)
                            ->withInput();
            }
    
            
            $product = Product::find($id);
            $order = new Order();
            $order->user_id = Auth::user()->id;
            $order->product_id = $id;
            $order->qty = $request['qty'];                                  
  this line $product->inv = $product->inv - $order->qty; (it works in store)
            $order->save();
            $product->save();
    
            return redirect('/myorders');
        
        }
    
    
            public function delete($id)
        {
            $order = Order::find($id);
            return view('order.myorders',compact('product','order','user'));
        }
    
    
              public function destroy(Request $request, $id)
            {
            $validator = Validator::make($request->all(), [
                'qty' => 'required|max:255',
                
            ]);
    
            if ($validator->fails()) {
                return redirect('')
                            ->withErrors($validator)
                            ->withInput();
            }
            $order = Order::find($id);
        $product= Product::find($order->product_id);
        $order->qty = $order->qty - $request->input('qty');
        $product->inv = $product->inv + $order->qty;
        $product->save();
        $order->save();
            return redirect('/myorders');
    
        }  
        
    }

and this is the routes file

Route::get('/myorders', 'OrderController@myorders');
Route::get('/add_order', 'OrderController@buy');
Route::get('/add_order/{id}', 'OrderController@store');
Route::get('/update_order', 'OrderController@edit');
Route::get('/update_order/{id}', 'OrderController@update');
Route::get('/delete_order', 'OrderController@delete');
Route::get('/delete_order/{id}', 'OrderController@destroy');

and this is my view, I am using it with

<div class="klaviyo_modal" id="update_order_qty" style="display:none;">
    <div class="klaviyo_inner">
        <a href="Cancel" class="klaviyo_close_modal klaviyo_header_close">×</a>
            <form method="GET" action="/update_order/{{ $order['id'] }}" enctype="multipart/form-data">
                {{ csrf_field() }}
                    <p class="klaviyo_header">Please update quantity as you need, then order it.</p>
                        <div class="klaviyo_fieldset">
                            <div class="{{ $errors->has('qty') ? ' has-error' : '' }}">
                                <label for="qty" style="display:block;text-align:center;margin-bottom:25px">Ordered New Quantity will be ADDED to your CURRENT Order Quantity</label>
                                    <input type="number" id="qty" class="qty" name="qty" style="display:block;margin:auto" required="required" placeholder="CURRENT Quantity">
                                    @if ($errors->has('qty'))
                                    <span class="help-block">
                                         <strong style="display:block;text-align:center">{{ $errors->first('qty') }}</strong>
                                    </span>
                                    @endif
                            </div>
                        </div>
          <div class="klaviyo_fine_print"></div>
              <div class="klaviyo_form_actions">
                 <button type="submit" class="klaviyo_submit_button">
                        <span>Update Order Quantity Now</span>
                 </button>
              </div>



            <!-- <span class="help-block">
                  <div class="success_message" style="display: block; text-align:center">Order Updated Successfully</div><br>
            </span>   
            <span class="help-block">  
                  <strong class="error_message" style="display: block; text-align:center">Sorry, Product Out of Stock, Try again later</strong>
            </span> -->        



            </form>
    </div>
</div>