Can't get scriptMenuAction to properly run (…the object no longer exists…)

Hi Loic, Have you tried adding a #targetengine at the top? This works for me in CC2021:

//targetengine for listeners
#targetengine "session";

var mySampleScriptAction = app.scriptMenuActions.add("Display Message");
var myEventListener = mySampleScriptAction.eventListeners.add("onInvoke",
    function () { alert("This menu item was added by a script."); });
//If the submenu "Script Menu Action" does not already exist, create it.
try {
    var mySampleScriptMenu = app.menus.item("$ID/Main").submenus.item("Script Menu Action");
    mySampleScriptMenu.title;
}
catch (myError) {
    var mySampleScriptMenu = app.menus.item("$ID/Main").submenus.add("Script Menu Action");
}
var mySampleScriptMenuItem = mySampleScriptMenu.menuItems.add(mySampleScriptAction);