Steps to reproduce the issue

• Download and install the latest Joomla! 4.x-dev branch.
• Open Article: Field Groups and create a new field group.
• Give it a title and hit "Save".

Expected result

The new group should have been saved.

Actual result

The save process fails with the following error message:

Error
Save failed with the following error: Incorrect datetime value: '' for column 'modified' at row 1

System information (as much as possible)

Windows NT DESKTOP-G94B9J7 10.0 build 15063 (Windows 10) i586
Database Version 5.5.5-10.1.25-MariaDB
Database Collation utf8mb4_unicode_ci
Database Connection Collation utf8_general_ci
PHP Version 7.1.7
Web Server Apache/2.4.26 (Win32) OpenSSL/1.0.2l PHP/7.1.7
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 4.0.0-dev Development [ Amani ] 31-March-2017 23:59 GMT
User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:54.0) Gecko/20100101 Firefox/54.0