PHP Core Warning – yii\base\ErrorException

$_SERVER = [
    'TMP' => '/srv/users/concorsige/tmp/concorsigruppoeventi',
    'TEMP' => '/srv/users/concorsige/tmp/concorsigruppoeventi',
    'TMPDIR' => '/srv/users/concorsige/tmp/concorsigruppoeventi',
    'PATH' => '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin',
    'USER' => 'concorsige',
    'HOME' => '/srv/users/concorsige',
    'SCRIPT_NAME' => '/index.php',
    'REQUEST_URI' => '/',
    'QUERY_STRING' => '',
    'REQUEST_METHOD' => 'GET',
    'SERVER_PROTOCOL' => 'HTTP/1.0',
    'GATEWAY_INTERFACE' => 'CGI/1.1',
    'REMOTE_PORT' => '53670',
    'SCRIPT_FILENAME' => '/srv/users/concorsige/apps/concorsigruppoeventi/public/index.php',
    'SERVER_ADMIN' => 'webmaster@',
    'CONTEXT_DOCUMENT_ROOT' => '/srv/users/concorsige/apps/concorsigruppoeventi/public',
    'CONTEXT_PREFIX' => '',
    'REQUEST_SCHEME' => 'http',
    'DOCUMENT_ROOT' => '/srv/users/concorsige/apps/concorsigruppoeventi/public',
    'REMOTE_ADDR' => '54.82.119.116',
    'SERVER_PORT' => '80',
    'SERVER_ADDR' => '127.0.0.1',
    'SERVER_NAME' => 'concorsi.gruppoeventi.org',
    'SERVER_SOFTWARE' => 'Apache/2.4.39 (Unix) OpenSSL/1.0.2g',
    'SERVER_SIGNATURE' => '',
    'LD_LIBRARY_PATH' => '/opt/sp/apache/lib:/opt/sp/apache/lib',
    'HTTP_ACCEPT_ENCODING' => 'gzip',
    'HTTP_IF_MODIFIED_SINCE' => 'Wed, 20 Mar 2019 01:05:13 GMT',
    'HTTP_ACCEPT_LANGUAGE' => 'en-US,en;q=0.5',
    'HTTP_ACCEPT' => 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
    'HTTP_USER_AGENT' => 'CCBot/2.0 (https://commoncrawl.org/faq/)',
    'HTTP_CONNECTION' => 'close',
    'HTTP_X_FORWARDED_FOR' => '54.82.119.116',
    'HTTP_HOST' => 'concorsi.gruppoeventi.org',
    'SCRIPT_URI' => 'http://concorsi.gruppoeventi.org/',
    'SCRIPT_URL' => '/',
    'FCGI_ROLE' => 'RESPONDER',
    'PHP_SELF' => '/index.php',
    'REQUEST_TIME_FLOAT' => 1555734487.121264,
    'REQUEST_TIME' => 1555734487,
    'argv' => [],
    'argc' => 0,
];

$_SESSION = [
    '__flash' => [],
];