Just yesterday my iphone 7 have been randomly crashing. I thought it was time to get a new one or send it to for warranty until my friend called and told me it was a bug. Went online and saw many posts on it. So the solution seems to be.

Change date

Stop date automatically update.

I think this one helps also.

Turn off ur notifications

Update software to 11.2

After update turn back all the settings that u desire.