-Using Terrain to mesh asset freezes the editor

-Steps to reproduce:
* Open Scenes/_TerrainTest
* Open the Terrain To Mesh tool. Window > VacuumShaders > Terrain To Mesh
* Drag the "Terrain" gameobject from the Hierarchy into the "Source Terrain" field in the TTM window.
* In the TTM window Enter the options as per the TTM Options screenshot attached
* Click "Generate"
* Wait for process to complete
* Editor will hang after "Setting up prefab" dialog finishes.
- It only occurs if the "Export trees" box is ticked. If not, it generates without any issue.
- If you successfully generate a mesh in the first time (for instance, without the "Export trees" option), then this bug doesn't happen.

The verification was made with 5.3.5f1(960ebf59018a), with iOS as the building platform. Couldn't try it on 5.4 because the asset doesn't work on beta versions.