Prestashop - unable to download PDF Invoices

I need a Prestashop Rock Star to help with an error which has just occurred after transferring my site to a new server. What I'm trying to do >> Download and print invoices from Prestashop admin Error encountered >> Notice: Trying to get property of non-object in /home/appleca/public_html/classes/[login to view URL] on line 702 Notice: Undefined variable: header in /home/appleca/public_html/classes/[login to view URL] on line 1076 FPDF error: Some data has already been output, cannot send PDF file Can you fix this quickly and without changing core files? Then please apply, thanks!