AssemblyObject::onChanged() was calling updateSolveStatus() when the
Group property changed during document restore. This triggered the
solver (solve -> validateNewPlacements) while child objects were still
being deserialized, causing a segfault in validateNewPlacements() due
to accessing uninitialized data.
Add isRestoring() and isPerformingTransaction() guards matching the
pattern used by GroupExtension::onChanged() and other FreeCAD modules.
9dc50cef72