Arch: Fix getCutVolume in ArchCommands.py

src/Mod/Arch/ArchCommands.py

@@ -427,7 +427,8 @@ def getCutVolume(cutplane,shapes,clip=False,depth=None):
         return None,None,None
     ce = p.CenterOfMass
     ax = p.normalAt(0,0)
-    u = p.Vertexes[1].Point.sub(p.Vertexes[0].Point).normalize()
+    prm_range = p.ParameterRange # (uMin, uMax, vMin, vMax)
+    u = p.valueAt(prm_range[0], 0).sub(p.valueAt(prm_range[1], 0)).normalize()
     v = u.cross(ax)
     if not bb.isCutPlane(ce,ax):
         #FreeCAD.Console.PrintMessage(translate("Arch","No objects are cut by the plane)+"\n")