Hello, I found the bug.
Noticed in the logs, a lot of new errors, noticed that one of my users unsuccessfully trying to ignore one of the thread, I checked it and got the same error, noticed that the Creator of the theme has long been removed. Repeated experiment, created a new user, created a new thread by this new user, then deleted user, and so same tried to ignore his subject. Got the same error. When you try to ignore themes created by remote users, you get an error.
Server error log
TypeError: Argument 1 passed to XF\Entity\User::canIgnoreUser() must be an instance of XF\Entity\User, null given, called in /var/www/ftp0/domain.net/httpdocs/src/addons/Truonglv/IgnoreContent/XF/Pub/Controller/Misc.php on line 82 src/XF/Entity/User.php:761
Generated by: MoSanders Feb 19, 2019 at 3:33 PM
Stack trace
#0 src/addons/Truonglv/IgnoreContent/XF/Pub/Controller/Misc.php(82): XF\Entity\User->canIgnoreUser(NULL)
#1 src/XF/Mvc/Dispatcher.php(321): Truonglv\IgnoreContent\XF\Pub\Controller\Misc->actionIgnoreContent(Object(XF\Mvc\ParameterBag))
#2 src/XF/Mvc/Dispatcher.php(248): XF\Mvc\Dispatcher->dispatchClass('XF:Misc', 'IgnoreContent', Object(XF\Mvc\RouteMatch), Object(Truonglv\IgnoreContent\XF\Pub\Controller\Misc), NULL)
#3 src/XF/Mvc/Dispatcher.php(100): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(Truonglv\IgnoreContent\XF\Pub\Controller\Misc), NULL)
#4 src/XF/Mvc/Dispatcher.php(50): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#5 src/XF/App.php(2177): XF\Mvc\Dispatcher->run()
#6 src/XF.php(390): XF\App->run()
#7 index.php(20): XF::runApp('XF\\Pub\\App')
#8 {main}