PHP Notice 'yii\base\ErrorException' with message 'Trying to get property of non-object' 

in D:\www\nenmenggu\frontend\views\layouts\main.php:69

Stack trace:
#0 D:\www\nenmenggu\frontend\views\layouts\main.php(69): yii\base\ErrorHandler->handleError(8, 'Trying to get p...', 'D:\\www\\nenmengg...', 69, Array)
#1 D:\www\nenmenggu\vendor\yiisoft\yii2\base\View.php(325): require('D:\\www\\nenmengg...')
#2 D:\www\nenmenggu\vendor\yiisoft\yii2\base\View.php(247): yii\base\View->renderPhpFile('D:\\www\\nenmengg...', Array)
#3 D:\www\nenmenggu\vendor\yiisoft\yii2\base\Controller.php(393): yii\base\View->renderFile('D:\\www\\nenmengg...', Array, Object(frontend\controllers\HelpController))
#4 D:\www\nenmenggu\vendor\yiisoft\yii2\base\Controller.php(379): yii\base\Controller->renderContent('<style>\n    .go...')
#5 D:\www\nenmenggu\frontend\controllers\HelpController.php(127): yii\base\Controller->render('view', Array)
#6 [internal function]: frontend\controllers\HelpController->actionView('14')
#7 D:\www\nenmenggu\vendor\yiisoft\yii2\base\InlineAction.php(55): call_user_func_array(Array, Array)
#8 D:\www\nenmenggu\vendor\yiisoft\yii2\base\Controller.php(154): yii\base\InlineAction->runWithParams(Array)
#9 D:\www\nenmenggu\vendor\yiisoft\yii2\base\Module.php(454): yii\base\Controller->runAction('view', Array)
#10 D:\www\nenmenggu\vendor\yiisoft\yii2\web\Application.php(84): yii\base\Module->runAction('help/view', Array)
#11 D:\www\nenmenggu\vendor\yiisoft\yii2\base\Application.php(375): yii\web\Application->handleRequest(Object(yii\web\Request))
#12 D:\www\nenmenggu\index.php(92): yii\base\Application->run()
#13 {main}