Following errors are thrown when generating materials for SpeedTree:

Assertion failed: Assertion failed on expression: '!GetPersistentManager().IsStreamLoaded(metaDataPath)'
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Asset import did not unload metadata path. This will leak memory. File a bug with repro steps please.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Regression from 2017b2.

Steps to reproduce:

See attached gif or:

1. Import Free SpeedTrees package to your project (or use the attached project)
2. Add terrain object
3. Add tree to terrain in Inspector (Edit Trees-> Add Tree)
4. Click on the tree prefab and make changes in Inspector (i.e. change color)
5. Click "Apply & Generate Materials"
6. Observe console