Cannot call DocumentApp.getUi() from a time based trigger

[Sneha Krishna Kumaran]

I'm trying to set up an alert that appears every 5 minutes on a google document. I've gotten the alert to show up by itself, but I'm running into the errors "Cannot call DocumentApp.getUi() from this context" whenever I run myFunction inside my time based trigger. Is there another way I can get an alert to pop up without using .getUI()? 

function createTimeDrivenTriggers() {

    // Trigger every at 5 min on document

    ScriptApp.newTrigger('myFunction')

    .timeBased()

    .after(1000*60)

    .create();

}

function myFunction() {

DocumentApp.getUi().alert("Please remember to update your progress towards your feedback goal.")

}

[Clark Lind]

You might try using a custom modal with html. Within the html file, you could use javascript. You might be able to set a JS timer to show/hide the modal. I'm not positive this is possible, but something you can experiment with.