Error: Attempt to assign property "first_version" on null

• /srv/code/openfun.app/lawtrace/libraries/DiffHelper.php:558
• /srv/code/openfun.app/lawtrace/controllers/LawController.php:332
• /srv/code/openfun.app/lawtrace/mini-engine.php:285
• /srv/code/openfun.app/lawtrace/mini-engine.php:249
• /srv/code/openfun.app/lawtrace/index.php:4