Hello,

The message is because the code is looking for visible column. But like you said, when you are in a form view, you have no columns.

So this module is done to work only with list view.

If you don't want to add the button in other view, you can update the JS file and add a condition in the redraw function : 

if (self.getParent().ViewManager.active_view === "list") {
                self.$el.find('.oe_sidebar').append(...
                self.$el.find('.oe_sidebar_printscreen_pdf')....
                self.$el.find('.oe_sidebar_printscreen_xls')....
}

It's maybe to check and to improve, but it's a first step :)

Good luck