Crash occurs when hovering over tooltips in ProBuilder advanced custom editor window.

Steps to reproduce:
1) Open the latest attached project (ProBuilder.zip) or import ProBuilder Advanced package from the Asset Store.
2) Go to Tools > ProBuilder > ProBuilder Window.
3) Start slowly hovering over the contents of the window (Vertex Colors, Handle: World, Center Pivot, etc.).

After going through 3-5 entries, a crash occurs in GUIView::RepaintAll(bool). This does not occur or Windows.
Reproduces with the Editor being in both OpenGL and Metal modes.

Reproduced on:
2017.1.0a1, 5.6.0b10, 5.6.0a4 (regression introduced here)

Did not reproduce:
5.6.0a3, 5.6.0a1, 5.5.2f1

Tested on:
macOS 10.12.4
Windows 10