[E_WARNING] A non-numeric value encountered

[FrederikP]

We upgraded from php7.0.27 to 7.3.6 and this error started showing up on admin panel.

ErrorException: [E_WARNING] A non-numeric value encountered src/addons/SV/WatchThreadsFilter/XF/Pub/Controller/Watched.php:417

Stack trace
#0 src/addons/SV/WatchThreadsFilter/XF/Pub/Controller/Watched.php(417): XF::handlePhpError(2, '[E_WARNING] A n...', '/share/www/foru...', 417, Array)
#1 src/addons/SV/WatchThreadsFilter/XF/Pub/Controller/Watched.php(448): SV\WatchThreadsFilter\XF\Pub\Controller\Watched->getWatchedThreadPrefixIds(Array, 7)
#2 src/addons/SV/WatchThreadsFilter/XF/Pub/Controller/Watched.php(153): SV\WatchThreadsFilter\XF\Pub\Controller\Watched->getWatchedThreadPrefixes(Array, 7)
#3 src/XF/Mvc/Dispatcher.php(321): SV\WatchThreadsFilter\XF\Pub\Controller\Watched->actionThreadsFilters(Object(XF\Mvc\ParameterBag))
#4 src/XF/Mvc/Dispatcher.php(244): XF\Mvc\Dispatcher->dispatchClass('XF:Watched', 'ThreadsFilters', Object(XF\Mvc\RouteMatch), Object(SV\WatchThreadsFilter\XF\Pub\Controller\Watched), NULL)
#5 src/XF/Mvc/Dispatcher.php(100): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(SV\WatchThreadsFilter\XF\Pub\Controller\Watched), NULL)
#6 src/XF/Mvc/Dispatcher.php(50): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#7 src/XF/App.php(2177): XF\Mvc\Dispatcher->run()
#8 src/XF.php(390): XF\App->run()
#9 index.php(20): XF::runApp('XF\\Pub\\App')
#10 {main}

Request state
array(4) {
  ["url"] => string(199) "/watched/threads/filters?unread=1&last_days=7&_xfRequestUri=%2Fwatched%2Fthreads%3Funread%3D1%26last_days%3D7&_xfWithData=1&_xfToken=1562282296%2C65b29045a3943234f12acd9a9a1d80b9&_xfResponseType=json"
  ["referrer"] => bool(false)
  ["_GET"] => array(7) {
    ["/watched/threads/filters"] => string(0) ""
    ["unread"] => string(1) "1"
    ["last_days"] => string(1) "7"
    ["_xfRequestUri"] => string(37) "/watched/threads?unread=1&last_days=7"
    ["_xfWithData"] => string(1) "1"
    ["_xfToken"] => string(43) "1562282296,65b29045a3943234f12acd9a9a1d80b9"
    ["_xfResponseType"] => string(4) "json"
  }
  ["_POST"] => array(0) {
  }
}