Sculpt overlay assumed that the sculpt session was always in sculpt mode. But the sculpt session was also used for Vertex/Weight painting. This resulted that when a mask was defined in the sculpt mode this resulted into render artifacts. This patch adds a check to see if the object has a sculpt session for OB_MODE_SCULPT. Reviewed By: Clément Foucault Differential Revision: https://developer.blender.org/D7704