Appropriate way to handling exiting/onBack in an app?

I apologize if this has been asked before - I didn't find anything on searching.

What is the appropriate implementation for app developers to ensure proper exiting/backing out of a watch app? Currently, when I load my app onto my Fenix 3 it works well but when I press the "Back" button to exit, it crashes. The code for my main BehaviorDelegate is below.

Also - a previous version of this code was not implementing onBack, and still crashed. Any help is appreciated - thanks.

(On a related note - what is the appropriate handling for the Up/Down keys in a widget to ensure that one can cycle to the adjacent widgets? Or should they be left un-handled?)


class LoopTimerDelegate extends Ui.BehaviorDelegate
{
hidden var app;

function initialize()
{
app = App.getApp();
}

function onMenu()
{
Ui.pushView(app.getNewSettingsMenu(), new LoopTimerMenuDelegate(), Ui.SLIDE_UP);

return true;
}

function onBack()
{
// Handle exiting
Ui.popView(Ui.SLIDE_RIGHT);
return true;
}

function onKey(evt)
{
var key = evt.getKey();
var handled = false;

if (WatchUi.KEY_ENTER == key)
{
app.toggleTimer();
handled = true;
}
else if (WatchUi.KEY_DOWN == key)
{
// Reset the timer
app.resetCurrentIteration();
handled = true;
}

if (handled)
{
Ui.requestUpdate();
}

return handled;
}
}