Joomla 3.8.12 and Abook 3.1.4 php 7.1.23
when editing a book in Admin panel 4 identical errors are shown :
Notice: Undefined index: data in /home/-----/public_html/administrator/components/com_abook/models/book.php on line 92
Looking at the file I see a comment above at line 91 which I think (according to google translate) says the next line is a fix for an issue you don't understand.
Looks like the issue must have been fixed in a recent Joomla update as if I simply comment out line 92 it all appears to work ok.
Thanks
RogerCO
This is a notice, I will fix it as soon as possible. but it's not a blocking error, if you want to avoid seeing it you can set the error level to "none" (global configuration->server->error reporting)