Returns the data that a router passed to the page in its response.
When you create a router and define its functionality in its router()
function, you can choose to send data along with the router's response.
That data is retrieved in the code of the page that was routed to using the
getRouterData() 関数である。
If you call the getRouterData() function from a non-router page or a router
page that wasn't sent any data, the function returns null.