symfony 3.4在null上调用成员函数getParameter（）

Question

我正在尝试获取'parameters.yml'中的值，例如

parameters:
    database_host: 127.0.0.1
    database_port: null
    database_name: symfony
    database_user: root
    database_password: null
    mailer_transport: smtp
    mailer_host: 127.0.0.1
    mailer_user: null
    mailer_password: null
    secret: 0b0d61852022c6b11009b968c2c1a7850b28ba72

Config.yml如下

doctrine:
    dbal:
        driver: pdo_mysql
        host: '%database_host%'
        port: '%database_port%'
        dbname: '%database_name%'
        user: '%database_user%'
        password: '%database_password%'
        charset: UTF8
        # if using pdo_sqlite as your database driver:
        #   1. add the path in parameters.yml
        #     e.g. database_path: '%kernel.project_dir%/var/data/data.sqlite'
        #   2. Uncomment database_path in parameters.yml.dist
        #   3. Uncomment next line:
        #path: '%database_path%'

    orm:
        auto_generate_proxy_classes: '%kernel.debug%'
        naming_strategy: doctrine.orm.naming_strategy.underscore
        auto_mapping: true

我想访问诸如database_host，database_port等的值， 但是我在这条线上出现错误：

public function __construct() {     
        $myparam = $this->getParameter('database_host');
}

Call to a member function getParameter() on null

我在这里犯什么错误，