I did notice a similar problem when I use the camera a lot: When I was on holidays and often took out my phone to take pictures, I had to restart it 2 or 3 times in a single day for this issue.
At some point the camera app (stock FP OS camera as well as GCam) only shows a black screen. Trying to switch to selfie camera crashes the app. I also found no other way except for a restart of the phone. It looks to me like some related software/driver component crashes at some point, but have not investigated further. It would be interesting to know if you can reproduce the issue and how often that happens.