App crash after a long time loading iOS12?
I had the same problem a few days ago. What fixed it was uninstalling the app and reinstalling it. 

Make sure that you save all your data first.

From this article:


  Press and hold the Top (or Side) button until the slider appears.
  
  Drag the slider to turn your device completely off.
  
  After your device turns off, press and hold the Top (or Side) button
  again until you see the Apple logo.