ok, got it. The problem is with main.BackButtonPressed.
When you click 'back' in your main activity, you exit the app, that's ok. BUT android doesn't always kill the process. Thus, when you go to 'KeyBoard' and the boolean backButtonPressed is true, then you perform an exitApp. This doesn't seem to have the desired results, and you get back to the editTickets activity at its resume event. Now, remember that you've just exited your app, so list is not found (not initialized) causing this error.
My suggestion (and I think Erel, Klaus and other friends here, would agree): When you start a new activity, close the previous one with activity.finish. This way, you will not have the need to use an exitApplication which can cause undesired effects as the error that occurs in your code right now. Personally, I had also initialization problems in a app I was developing, but when I did this, such errors completely disappeared. Hope this helps.