Steps to reproduce:

1. Open ScrollRectErrorReduced project
2. Open UI-scene scene
3. Press Play
4. Press Missions button
5. Observe console

Expected outcome: UI Mesh renders, scripts throw no errors, animations work fine.
Actual outcome: Whenever the "Gradient" script is triggered, console throws Mesh.uv3 is out of bounds errors (And mesh.uv4 as well)

Reproduced on 5.6.0f3 , 5.6.0f1 , 5.6.0b8, 5.6.0b6
Works fine on 5.5.3f1 , 5.6.0b3 , 5.6.0b4
Regression introduced in 5.6.0b5

Note: The error is caused by the Gradient script. You can find it attached to UI graphic elements such as Canvas->Missions->Dialog->Missions->Mission01->Description If you edit the script and go to line 63, where the "vertexHelper.AddUIVertexTriangleStream" is, and disable it, the error disappears. Error is not found in earlier versions of Unity even with the line left active.