Steps to reproduce:
1. Open the attached project
2. In top menu, click Tools->ProBuilder Basic->ProBuilder Basic Window
3. In the newly opened UI window, click "New Shape"
4. Click anywhere on the "New Shape" window

Result:

• New Shape window is colored red instead of default UI color
• Clicking anywhere on the window creates a few empty GameObjects "ProBuilder Mesh"
• Errors in console are shown:

MissingMethodException: Method not found: 'UnityEngine.Mesh.Clear'.
GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced)

• Failed rebuilding null pb_Object. Cached mesh attributes are invalid or missing.

Reproduced on: 2017.2.0b2
Did not reproduce on: 2017.1.0f3