Not creating the bibliography

Hello!

Google docs is giving me this message when I try to generate a bibliography :
A Google Apps Script error occurred with message “Cannot read property “getNumChildren” from null.”. Contact Paperpile support to resolve this issue.

One or more of these errors caused processing to not be completed.

What is going on? Once you guys can show me that the App works, I will be happy to get a subscription…

THANK YOU!
Shahar