Wordpress: add_submenu_page() creating error - wordpress

Question

When I try to create a main menu, with this line
add_menu_page("page title","menu name",10,"test-slug");
Wordpress loads just fine and displays the new menu, but when I try to add a submenu,
add_menu_page("page title","menu name",10,"test-slug");
add_submenu_page("test-slug","sub title","sub menu",10,"test-sub-slug");
only an empty page is displayed, right after half a second. Seems like a php-error, but I can't see the error. Does anyone see what I am doing wrong?

Answer 1

Try the code below in your functions.php file. It should work correctly.
add_menu_page(
'Menu Page Title', // page title
'Menu Menu Text', // menu link text
'manage_options', // capability to access the page
'menu_slug', // page URL slug
'menu_callback_function', // callback function to display the content on options page
'dashicons-format-status', // menu icon
2 // priority
);
add_submenu_page(
'menu_slug', // page URL slug
'Sub Menu Title', // page title
'Sub Menu Text', // menu link text
'manage_options', // capability to access the page
'submenu_slug', // page URL slug
'submenu_callback_function', // callback function to display the content on options page
1 // priority
);
function menu_callback_function(){
echo "This is menu page...";
}
function submenu_callback_function(){
echo "This is submenu page...";
}

Answer 2

Problem solved. add_submenu_page() obviously must be called using a hook. Hence, this way it works and will not cause Wordpress to crash:
function dp_menu_item()
{
add_menu_page("page title","menu name","manage_options","mypage.php");
add_submenu_page("edit.php?post_type=cpt_people","sub title","sub menu","manage_options","mysubpage.php");
}
add_action("admin_menu", "dp_menu_item");