Hi!
The documentation for c4d.utils.SendModelingCommand
states
doc (c4d.documents.BaseDocument) – The document for the operation. Should be set if possible. Must be set for MCOMMAND_JOIN [...]
But this code for example works fine:
splineA = c4d.BaseObject(c4d.Osplinecircle)
splineB = c4d.BaseObject(c4d.Osplinecircle)
splineB[c4d.PRIM_PLANE] = c4d.PRIM_PLANE_XZ
null = c4d.BaseObject(c4d.Onull)
splineA.InsertUnder(null)
splineB.InsertUnder(null)
splineC = c4d.utils.SendModelingCommand(c4d.MCOMMAND_JOIN, [null], doc=None)[0]
doc = c4d.documents.GetActiveDocument()
doc.InsertObject(splineC)
Is there a specific reason you should have a document to do that operation in?
On a side note it would be really helpful if the documentation stated that you need to have the objects in a null in order to merge them and not pass a list of the objects as you might think.