Steps to repro:

1. Open attached project and 't1' scene;
2. Select 'Terrain' gameobject in the Hierarchy;
3. In the Inspector go to 'Paint Trees' tab;
4. Select 'Sphere' prefab in 'Paint Trees' ssection in Inspector;
5. Click 'Mass Place Trees' button;
6. Observe opened 'Place Trees' popup window.

Expected result:
'Place Tree' popup window contains 2 UI elements: Number of trees (text and textbox) and Keep Existing Trees checkbox (see attached expected.png screenshot.

Actual result:
'Place Trees' popup window has additional weird 'Persistent View Data' field which accepts object of Serializable JSON Dictionary type. (see attached actual.png screenshot.
Regression introduced in Unity 2017.3.0a7.

Note:
- Reproducible in 2017.3.0f2, 2017.3.0a7;
- Not reproducible in 2017.3.0f6;
- Reproducible on Windows 10 and OSX 10.12.6.