UI API called on a background thread

[felipeCostaNascimento]

I got an error message when function "thumbnailCount" is called:

Main Thread Checker: UI API called on a background thread: -[UIView frame][...]

Apparently, just calling this function inside the main thread seems to eliminate the error message.

I'm running on Swift 4, Xcode 9.2.

[jacolack]

I'm getting the same error, has anyone else been able to silence it with actual code? I found an answer here but I don't know how to actually solve the issue.