Error: Attempt to modify property "對照表" on null

• /srv/code/openfun.app/lawtrace/libraries/DiffHelper.php:168
• /srv/code/openfun.app/lawtrace/controllers/LawController.php:260
• /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