If you want to save yourself a lot of time trying to track down your app just quitting in Xojo 2018. Check your constraints on ALL of your controls. I would recommend editing each constraint entry within Xojo 2018. You don’t have to make any changes, just pull it up to display and then click done.
I recommend this because my main app would just quit upon trying to Dimension my main page. Thanks to Travis we finally determined that there was an illegal constraint on two of my controls. The Top constraint that was attached to Parent.Left instead of Parent.Top.
The issue was fixed by just editing the Top constraint and closing it. I didn’t have to change the value. It changed to Top by just editing it. It will only fix it in a Xojo 2018 version though so make sure that is what you have your application loaded in.