Steps to reproduce:
1. Open attached project's scene "Test".
2. Exit Unity Editor.
3. Editor crashes.

Crash is caused by HideFlags.DontSave:

[ExecuteInEditMode]
public class Test : MonoBehaviour
{
public void OnEnable()
{
var data = new TerrainData();
data.hideFlags = HideFlags.DontSave;
}
}

Reproduced on Windows 10 (64 bit) with 5.3.0b3 (b6181bece87a), 5.2.2f1 (f5cc61d36935), 5.2.0f2 (d7f8e5b3cbb5b)

Fix verified on 5.4.0a4 (426f677d5fa7)