As far as I know Assertion failed on expression: 'Thread::CurrentThreadIsMainThread()' is debug information and it should not appear in releases builds.

Steps to reproduce
- Open CubeShooterWithNavMesh project from OwnCloud https://oc.unity3d.com/index.php/s/6XuS8zACwh828G6
- Select the NavMesh Object in the hierarchy
- In the inspector find the Nav Mesh Surface script
- Click "Bake"
Notice issue >> Assertion failed on expression: 'CurrentThreadIsMainThread()' spews to the console.

NOTE:
I don't think this has anything to do with the Nav Mesh as it happens eventually without it. The above is a simple way to reproduce the issue.

Broken in 2018.3.0a11, 2018.3.0a10