fix: cast first argument of calls to getProduct() to int (productId)

See merge request !17

Dev

See merge request !15

fix: consider 0% as vat for the netto price calculation

See merge request !14

fix: erp output consider empty default template settings

See merge request !12

fix: PHP Fatal error:  Uncaught TypeError: QUI\ERP\Comments::__construct(): Argument #1 ($comments) must be of type array, null given