Output has been emitted automatically

SHC

I'm getting an error whenever I try to create a post saying this error message:

Warning: count(): Parameter must be an array or an object that implements Countable in /home/stpl/jgo-flarum/vendor/flarum/core/src/Core/Post/RegisteredTypesScope.php on line 52

Fatal error: Uncaught RuntimeException: Output has been emitted previously; cannot emit response in /home/stpl/jgo-flarum/vendor/zendframework/zend-diactoros/src/Response/SapiEmitterTrait.php:31
Stack trace:
#0 /home/stpl/jgo-flarum/vendor/zendframework/zend-diactoros/src/Response/SapiEmitter.php(27): Zend\Diactoros\Response\SapiEmitter->assertNoPreviousOutput()
#1 /home/stpl/jgo-flarum/vendor/zendframework/zend-diactoros/src/Server.php(162): Zend\Diactoros\Response\SapiEmitter->emit(Object(Zend\Diactoros\Response))
#2 /home/stpl/jgo-flarum/vendor/flarum/core/src/Http/AbstractServer.php(36): Zend\Diactoros\Server->listen(Object(Zend\Stratigility\NoopFinalHandler))
#3 /home/stpl/jgo-flarum/index.php(16): Flarum\Http\AbstractServer->listen()
#4 {main} thrown in /home/stpl/jgo-flarum/vendor/zendframework/zend-diactoros/src/Response/SapiEmitterTrait.php on line 3

SHC

I'm using the latest Flarum install by the way. I tried reinstalling flarum from scratch, but got the same issue afterwards.

Ralkage

SHC This is mostly an issue with PHP 7.2.x as Flarum does not yet support this version of PHP.

Just to verify, which PHP version are you currently using?

SHC

Ah that must be it. I'm using 7.2.1 that comes with XAMPP on Linux. Is there any fix for it? I see from another post that it is already fixed in upstream. Can I get it somehow for testing purposes?