You MAY call the load model thing in simpleInitApp() as you did in your snippet. You cannot call it from main(), which makes little sense to do anyway as it gives you nothing extra, so stay in simpleInitApp() or call the loadModel function from an update loop using any trigger to keep it from spawning thousands of instances, as stated above.
I suspect the nullPointerException is not complaining about the model obj but about the assetManager, judging by the fact you are trying to use that while the app is not fully started yet and it being the big difference between loadBox and loadModel. You cannot use that assetManager before it is initialized.
float totalTime = 0 ; //voila now holds totalTime from when application started.
simpleUpdate(tpf)
{
totalTime += tpf;
}[/java]
You need tpf, because some computers run slow e.g 30 frames per seconds, while others run 60000 frames per second. If u dont use tpf, then fast computers will move at omega speed and make game unplayable.