Blender Git Loki

Ensure dependency graph exists before initializing editors

Will help entering sculpt mode on file load by making it possible
to fully initialize sculpt session. The goal is to make sure PBVH
exists since the very beginning of file open (missing PBVH is a
reason why object is not visible before first stroke).

This is not enough yet to fully solve the issue, since entering
sculpt mode tags object for Copy-on-Write update, which frees
PBVH.