Xcode message - method should not be called UI unresponsiveness

Running some final tests before release. In Xcode, when testing on a device, I'm seeing constant stream of this message:

This method should not be called on the main thread as it may lead to UI unresponsiveness.

If that link doesn't work, here's another:
https://imgur.com/a/4yvYzIf

Is there something that's causing the message? I haven't made any source code changes.