Ingame bge addObject and add or use a new modifiers

Coding Python Support
1

In game, I will spawn an Object and change like SUBSURF or Armature modifier.
Why can I not add or use a modifiers? (no errors)

templateName = 'template Object'
first = bge.logic.getCurrentScene().addObject(templateName,  bge.logic.getCurrentController().owner, 0)
subsurf_modifier = first.blenderObject.modifiers.new(name="Subdivision Surface", type='SUBSURF') # later i change "SUBSURF" to Armature
print(subsurf_modifier) # Why is this None?