1 extra detail (could be important):
I use Xcode 8.2.1.
In Build settings I had to change ‘Use Legacy Swift Language Version’ from ‘Unsupported version: 4.0’ to—-> ‘No’
Otherwise the project would not build.
Could this be the cause?
But still, it did work for a while when I began setting the project up, changing stickers, adding the pattern, etc.