Steps to Repo:
1. Hierarchy > Right-click > 3D Object > Terrain
2. Terrain Component > Paint Trees > Edit Trees > Add Tree
3. Delete tree asset and prefab used by terrain > Select terrain in scene
4. Make sure you are on the 'Paint Trees' tab, try select the missing tree in the inspector
5. Observe crash

Actual Result:
Editor crashes

Expected Result:
Editor shouldn't crash and the 'Paint Tree' setting should notify for the missing prefab

Frequency:
Always

Not reproducible with:
2017.1.0f3

Reproducing in:
2017.30b9, 2017.2.0f3, 2017.3.0b1, 2017.3.0f1

Environment:
2017.30b9
Windows