415 Unsupported Media Type

An Error occurred while handling another error:
yii\web\HeadersAlreadySentException: Headers already sent in /public_html/bwcms/common/components/Collector.php on line 65. in /public_html/bwcms/vendor/yiisoft/yii2/web/Response.php:368
Stack trace:
#0 /public_html/bwcms/vendor/yiisoft/yii2/web/Response.php(341): yii\web\Response->sendHeaders()
#1 /public_html/bwcms/vendor/yiisoft/yii2/web/ErrorHandler.php(136): yii\web\Response->send()
#2 /public_html/bwcms/vendor/yiisoft/yii2/base/ErrorHandler.php(135): yii\web\ErrorHandler->renderException(Object(yii\base\ErrorException))
#3 [internal function]: yii\base\ErrorHandler->handleException(Object(yii\base\ErrorException))
#4 {main}
Previous exception:
yii\base\ErrorException: Cannot modify header information - headers already sent by (output started at /public_html/bwcms/common/components/Collector.php:65) in /public_html/bwcms/frontend/web/RunSitemap.php:76
Stack trace:
#0 [internal function]: yii\base\ErrorHandler->handleError(2, 'Cannot modify h...', '/public_html/bw...', 76)
#1 /public_html/bwcms/frontend/web/RunSitemap.php(76): header('Content-type: t...')
#2 /public_html/bwcms/frontend/web/RunSitemap.php(32): frontend\web\Sitemap->__construct()
#3 /public_html/bwcms/vendor/yiisoft/yii2/base/BaseObject.php(109): frontend\web\RunSitemap->init()
#4 /public_html/bwcms/vendor/yiisoft/yii2/base/Application.php(204): yii\base\BaseObject->__construct(Array)
#5 /public_html/bwcms/frontend/web/sitemap.php(26): yii\base\Application->__construct(Array)
#6 {main}