I've got the same error on a few extensions (gamification & poll). It causes the page stuck and didn't refresh but it works after a manual refresh. I tried clear the cache, re-enabled the extension and reinstalled flarum but non-working.
PHP 7.1
POST http://127.0.0.1/forum/api/discussions
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<b>Notice</b>: Undefined index: poll in <b>D:\site\flarum\vendor\treefiction\polls\src\Listeners\SavePollToDatabase.php</b> on line <b>34</b><br />
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<b>Fatal error</b>: Uncaught RuntimeException: Output has been emitted previously; cannot emit response in D:\site\flarum\vendor\zendframework\zend-diactoros\src\Response\SapiEmitterTrait.php:31
Stack trace:
#0 D:\site\flarum\vendor\zendframework\zend-diactoros\src\Response\SapiEmitter.php(27): Zend\Diactoros\Response\SapiEmitter->assertNoPreviousOutput()
#1 D:\site\flarum\vendor\zendframework\zend-diactoros\src\Server.php(162): Zend\Diactoros\Response\SapiEmitter->emit(Object(Flarum\Api\JsonApiResponse))
#2 D:\site\flarum\vendor\flarum\core\src\Http\AbstractServer.php(36): Zend\Diactoros\Server->listen(Object(Zend\Stratigility\NoopFinalHandler))
#3 D:\site\flarum\api.php(16): Flarum\Http\AbstractServer->listen()
#4 {main}
thrown in <b>D:\site\flarum\vendor\zendframework\zend-diactoros\src\Response\SapiEmitterTrait.php</b> on line <b>31</b><br />