Spinning wheel prevents mouse input to cancel script (SUI)

I don't know about the spinning wheel, I don't see that on my Windows PC. Do you use a Mac?

For the cancel button, your code still works its way through the whole file array, it just doesn't do anything. Maybe change the cacel burron's function like this:

buttonp.onClick = function() {
  // Close all documents, then
  exit();
}

By the way, it looks as if there's a bug in asynchronousExportFile(). When you use it, document.close() returns an error. The error disappears when you use the synchronous exportFile(). The latter works fine, your script runs ok over here (with exportFile()).

On another note, like Loic I was surprised at your forEachDescendantFile() function. As I understand it, you open, export, and close each document inside a recursive function. That looks overwrought to me, and may explain why you run into difficulties that require your Monty Pythonesque error message ""The object at . . . is a child of a folder and yet is not a file or folder."

To find files in a folder and its subfolders and process them, this seems much simpler:

function findIndd (f, list) {
  var f = Folder(f).getFiles('*.*');
  for (var i = 0; i < f.length; i++) {
    if (f[i] instanceof Folder) {
      findIndd (f[i], list);
    } else if (/\.indd$/i.test (f[i].name)) {
      list.push (f[i]);
    }
  }
  return list;
}


indds = findIndd (myFolder, []);
for (i = 0; i < indds.length; i++) {
  // process file
}

 Just a thought. . .

Thanks for your reply. I changed to the asynchronous method but the Cancel button is still not clickable.

By default yes if you do export syncronously where you basically freeze the app (it's a modal process). If you use asynchronousExportFile You will have exports done in the background with the app non frozen (non modal). This will create background tasks that you can possibly monitor and "possibly" stop processes  (tbh I am not completely sure but that's where I would investigate).