old_contents: import pyblish.api
  
  
  class ValidateMindbenderSingleAssembly(pyblish.api.InstancePlugin):
      """Each asset must have a single top-level group
  
      The given instance is test-exported, along with construction
      history to test whether more than 1 top-level DAG node would
      be included in the exported file.
  
      """
  
      label = "Validate Single Assembly"
      order = pyblish.api.ValidatorOrder
      hosts = ["maya"]
      families = ["mindbender.model", "mindbender.rig"]
  
      def process(self, instance):
          from maya import cmds
          from mindbender import maya
  
          with maya.maintained_selection():
              cmds.select(instance, replace=True)
              nodes = cmds.file(
                  constructionHistory=True,
                  exportSelected=True,
                  preview=True,
                  force=True,
              )
  
          assemblies = cmds.ls(nodes, assemblies=True)
  
          assert len(assemblies) == 1, (
              ("Multiple assemblies found."
                  if len(assemblies) > 1
                  else "No assembly found")
          )
  
new_contents: import pyblish.api
  
  
  class ValidateMindbenderSingleAssembly(pyblish.api.InstancePlugin):
      """Each asset must have a single top-level group
  
      The given instance is test-exported, along with construction
      history to test whether more than 1 top-level DAG node would
      be included in the exported file.
  
      """
  
      label = "Validate Single Assembly"
      order = pyblish.api.ValidatorOrder
      optional = True
      hosts = ["maya"]
      families = ["mindbender.model", "mindbender.rig"]
  
      def process(self, instance):
          from maya import cmds
          from mindbender import maya
  
          with maya.maintained_selection():
              cmds.select(instance, replace=True)
              nodes = cmds.file(
                  constructionHistory=True,
                  exportSelected=True,
                  preview=True,
                  force=True,
              )
  
          assemblies = cmds.ls(nodes, assemblies=True)
  
          assert len(assemblies) == 1, (
              ("Multiple assemblies found."
                  if len(assemblies) > 1
                  else "No assembly found")
          )
  
subject: add optional true temp fix for #49