yii2 HeadersAlreadySentException 報錯

yii\web\HeadersAlreadySentException: Headers already sent in D:\AppServ\www\qstv\yii-basic\controllers\IndexController.php on line 29. in D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\web\Response.php:366
Stack trace:
#0 D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\web\Response.php(339): yii\web\Response->sendHeaders()
#1 D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\web\ErrorHandler.php(135): yii\web\Response->send()
#2 D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\base\ErrorHandler.php(111): yii\web\ErrorHandler->renderException(Object(yii\web\HeadersAlreadySentException))
#3 [internal function]: yii\base\ErrorHandler->handleException(Object(yii\web\HeadersAlreadySentException))
#4 {main}
Previous exception:
yii\web\HeadersAlreadySentException: Headers already sent in D:\AppServ\www\qstv\yii-basic\controllers\IndexController.php on line 29. in D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\web\Response.php:366
Stack trace:
#0 D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\web\Response.php(339): yii\web\Response->sendHeaders()
#1 D:\AppServ\www\qstv\yii-basic\vendor\yiisoft\yii2\base\Application.php(392): yii\web\Response->send()
#2 D:\AppServ\www\qstv\yii-basic\web\index.php(12): yii\base\Application->run()
#3 {main}

這是有輸出的時候,沒有加 exit 致使的