Fatal error: Uncaught Error: Call to a member function on() on null in

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Fatal error: Uncaught Error: Call to a member function on() on null in E:\phpStudy\PHPTutorial\WWW\api.php:10 Stack trace: #0 E:\phpStudy\PHPTutorial\WWW\bootstrap.php(309): include() #1 E:\phpStudy\PHPTutorial\WWW\index.php(22): require('E:\\phpStudy\\PHP...') #2 {main} thrown in E:\phpStudy\PHPTutorial\WWW\api.php on line 10