old_contents: 
  def add_current_unit(request):
      """Template context processor to add current unit"""
  
      return {'CURRENT_UNIT': get_current_unit(request)}
  
  
  def get_current_unit(request):
      """Return the current unit"""
  
      from units.models import Unit
  
      current_unit_pk = request.session.get('current_unit_pk', 1)
  
      try:
          current_unit = Unit.objects.get(pk=current_unit_pk)
      except Unit.DoesNotExist:
          current_unit = Unit.objects.get(pk=1)
  
      return current_unit
  
  
  def update_current_unit(request, unit_pk):
      """Update the current unit"""
  
      request.session['current_unit_pk'] = unit_pk
  
new_contents: from django.conf import settings
  
  def add_current_unit(request):
      """Template context processor to add current unit"""
  
      return {'CURRENT_UNIT': get_current_unit(request)}
  
  
  def get_current_unit(request):
      """Return the current unit"""
  
      from units.models import Unit
  
      current_unit_pk = request.session.get('current_unit_pk', 1)
  
      try:
          current_unit = Unit.objects.get(pk=current_unit_pk)
      except Unit.DoesNotExist:
          try:
              current_unit = Unit.objects.get(pk=settings.ROOT_UNIT_PK)
          except:
              current_unit = None
  
      return current_unit
  
  
  def update_current_unit(request, unit_pk):
      """Update the current unit"""
  
      request.session['current_unit_pk'] = unit_pk
  
subject: Fix error if no units