app.open() is not a function

Using ExtendScript to develop with After Effects CC 2018.  The script worked in the development environment until a few days ago.

Now I have the error app.open is not a function.  And it is right: looking through the data browser, app refers to the ExtendScript Toolkit rather than AfterEffects.

The scrap of script below uses a working project file which I can load manually but cannot find a way to load through a script in ExtendScript debugging.

Suggestions gratefully received!

I am running:

Adobe After Effects CC 2018

ExtendScript Toolkit 4.0.0.1

Here is the nub of the issue in code - it really does not get more basic than this.

var my_file = new File("/Users/rl/Documents/Dev/AdobeData/candidates/tamara/tamararig.aep");

if (my_file.exists) {

    app.open(my_file); //fails on this line with app.open is not a function

}