I just got my Spark back from the DJI repair facility and took it out for a flight today and the app started crashing after about 15 min of flight. I was on my second battery and it had worked fine until then but I couldn't get the app to load after that. It would load and freeze and crash and load and freeze and crash. Completely ruined any chance I had to get any footage. Has anybody else had any experience with this and have any tips? My app is up to date and my phone is an iPhone 6S running iOS 11.3. So frustrating

Try these tips:
Install DJI GO on an Apple Tablet or Phone (Step-by-Step Guide)

Update IOS to the latest version and reinstall the app.

Sorry to hear the difficulties. Please try to reinstall the latest app, note to back up the important files and sync the flight records before reinstalling it. If the app still keeps crashing, please provide me with the info below for further checking.
Info needed: the version of the app and your DJI account, as well as the crash logs. The crash logs of the app need to be exported by iTools, please install iTools on your computer----Toolbox----Advanced Features.
Appreciate your help.