What happened
Editor crashes when clicked on Hierarchy/Project window while Add Tree/Add Detail Mesh Window appears.

Steps to Reproduce:
1. Create a New project
2. Add Terrain from Gameobject > 3D Objects > Terrain
3. Click on Add Paint Detailsl > Edit Details > Add Detail Mesh
4. "Add Detail Mesh" window appears.
5. Close the "Add Detail Mesh" window
6. Click on Hierarchy/Project window
Editor crashes

OR

1. Create a New project
2. Add Terrain from Gameobject > 3D Objects > Terrain
3. Click on Add Tree > Edit Tree > Add Tree
4. "Add Tree" window appears
5. Click on Hierarchy/Project window
Editor crashes

Environment:
- Appears on both platforms (Mac and Windows).

Frequency:
- Appears Always.

Reproducible on: 2018.1.0a7, 2017.3.0f2
Working Fine on: 2018.1.0a5, 2017.3.0f1, 2017.2.0f3