ya prove usando print_r(variable) y me da vacio, pero ya no se de donde viene el error y lo me lo marca en esta variable

$amountArray[$idproduct] += $amount;

, como pudiera solucionarlo

 function saveAllSaleDetails($idsale, $sale) {
			$this->conexion->startTransaction();
			$amountArray = [];
			try {
				foreach ($sale as $detail):
					$idproduct = $detail['id'];
					$amount = $detail['amount'];
					$price = $detail['price'];
					$subtotal = $detail['subtotal'];
					$iduser = 1;
					$this->saveSaleDetail($idsale, $idproduct, $amount, $price, $subtotal, $iduser);
					$amountArray[$idproduct] += $amount;
					$stock = $this->product->getProductStock($idproduct);
					$stock = $stock[0][0] - $amountArray[$idproduct];
 
					if ($stock <= 20) {
						$product = $this->product->getProductById($idproduct);
						$message = $product[0][1]." stock is bellow 20.";
						notification::add($message, $idproduct, 'warning', 'product.php');
					}
				endforeach;
 
				$this->conexion->commit();
				$this->conexion->cerrar();
				return true;
 
			} catch (Exception $e) {
				$this->conexion->rollback();
				$this->conexion->cerrar();
				var_dump($e->getMessage());
				return false;
			}
 
		}