Steps to reproduce:
1. Select "New Material" in the project
2. The inspector shows the "Standard" shader editor
3. Change the shader to "Nature/SpeedTree"
4. The inspector does not show the "SpeedTree" editor
5. Deselect and reselect the "New Material" editor
6. The inspector shows the "SpeedTree" editor

Expected result: custom material editor should be refreshed when changing shader type

Reproduced in: 2017.3.0a7, 2017.2.0b11, 2017.1.1p2, 5.6.3p3, 5.6.0a1
Does not reproduce in: 5.5.4p4, 5.4.6f3
Regression since: 5.6.0a1