Hi Eto Yutaro,
You are getting null because you haven’t configured the Application Root URL. To configure it you need to set the configuration value in either Mendix service Console or the Environment Details for your application environment, depending on how you are deploying your Mendix application:
In the Mendix Service Console (on premises):
Or in the Environment Details from your application:
Make sure you add your application URL (both configuration name and value are case sensitive) here as a new runtime configuration, more information here: