Caused by {rB3b6ee8cee708}
Above commit was trying to get the vertexgroup from the mesh that is
passed into `deformVertsEM` (but that can be NULL).
When can it be NULL, when is is non-NULL?
`editbmesh_calc_modifiers` only passes in a non-NULL mesh to
`deformVertsEM` under certain conditions:
- a non-deform-only modifier is handled currently
- a non-deform-only modifier preceeds the current modifier
- a deform-only modifier preceeds the current modifier (and the current
one depends on normals)
So the passed-in mesh cannot be relied on, now get the vertex group from
the context object data (like it was before the culprit commit).
Related commit: rB8f22feefbc20
Maniphest Tasks: T93611
Differential Revision: https://developer.blender.org/D13487
0e52af097f