erp5_document_scanner: fix desactivation of the new page button

bt5/erp5_document_scanner/PathTemplateItem/web_page_module/scanner_gadget_document_scanner_js.js

@@ -395,7 +395,7 @@
         thumbnail_dom_list.push(domsugar('button', {type: 'button',
                                                     text: result_list[0],
                                                     // Do not allow to show again the current image
-                                                    disabled: (len === gadget.state.display_index - 1),
+                                                    disabled: (gadget.state.display_index === null) || (len === gadget.state.display_index - 1),
                                                     "class": 'new-btn ui-btn-icon-left ui-icon-plus'
                                                    }));
         return domsugar('ol', {"class": "thumbnail-list"}, thumbnail_dom_list);