window.open might return null

[dtsamitros]

https://github.com/DataTables/Buttons/blob/445197cdb0bcbef2d1eff83844e6624d4a5151d6/js/buttons.print.js#L139-L140

I am using the latest version of Firefox and is by default blocking popups (https://developer.mozilla.org/en-US/docs/Web/API/Window/open#return_value) and a console error is thrown consequently (Uncaught TypeError: win is null). Perhaps a callback eg windowOpenFailed should be provided so the client can handle the error, or perhaps a warning should be displayed to the user.

[DataTables]

Thanks for flagging this issue up! Fix committed in master 1f0ebf5.