[Edit: Update at bottom]
Working on updating my project to 2019.3.
Here is what I did:
1) Updated Unity project to 2019.3.0b12
2) Updated Spine Unity Runtime to the 3.9-beta version
3) Updated Timeline UPM package to the 3.9-beta version
Error in Editor:
Code section:
Animation State variable declaration:
It's quite possible that I did something wrong when updating, but I've tried removing/re-adding both the Spine Runtime and the Timeline package, and still get this error.
Additional bit of info, I don't think this matters but the Timeline 3.9-beta shows up in the package manager as 3.8.1. I triple-checked that I'm installing the 3.9-beta version:
Update: I switched everything (Spine Runtime & Timeline UPM) back to 3.8 and everything looks to be good!