Backwards compatibility with WC 2.6.14

A question was asked by Malte Vollmerhausen (malte.vollmerhausen@bindit.de)

Dear Mr. Kainhofer, after installing the newest update (v1.4.1) of Advanced Order Numbers for WooCommerce on WooCommerce v2.6.14 we get internal server errors when generating invoice PDFs through the PDF Invoices and Packing Slips plugin and new order numbers are not correctly created. This seems to be caused by using a WooCommerce 3.x only method without an appropriate compatibility check for older WooCommerce versions. Since we cannot yet update to WooCommerce 3.x because of other incompatibilities, it would be great if you could take a look at this issue. I can also provide a more detailed error description (File and line where the error occurs) if necessary. With kind regards, Malte Vollmerhausen