Wow. Thanks TIG and sdmitch, that works!
But why might that be? I would think that making a new component definition without yet any entities would not be that fragile or incomplete. Is there any documentation that states this, or suggests this? Why isn't this like declaring a variable but not initializing it? It also would have helped if the error message indicated which component definition it didn't like. Is this a ruby thing or a Sketchup thing? Thanks.

I'm trying to add an instance of a component definition as an entity in another component definition, but I'm getting an error. I searched here but didn't find anything that helped yet. Any thoughts or guidance on how to diagnose and fix? Code and error below, with error line marked. Thanks.

def self.create_component_definition_piece(width, height, depth, angle)
  @componentDefinitionPiece=@definitions.add "Piece"
  ptLL=[0,0,0]
  ptLR=[width,0,0]
  ptUR=[width,0,height]
  ptUL=[0,0,height]
  new_face = @componentDefinitionPiece.entities.add_face ptLL, ptLR, ptUR, ptUL
  if new_face.normal == [0,-1,0]
    new_face.pushpull depth
  else
    new_face.pushpull -depth
  end
end

def self.create_component_definition_laddershelves()
  @componentDefinitionLadderShelves=@definitions.add "LadderShelves"
  self.create_component_definition_piece(2.5,48,0.75,0)
  @componentDefinitionLadderShelves.entities.add_instance(@componentDefinitionPiece,@transform000)     <<<<< Error on this line.
end

# Draw the ladder shelves
self.create_component_definition_laddershelves()
transform=Geom;;Transformation.new([0,0,0])
rotateAndTransform=transform * Geom;;Transformation.rotation([0,0,0],[0,1,0],0)
@entities.add_instance(@componentDefinitionLadderShelves,rotateAndTransform)
# puts "Number of entities in LadderShelves=",@componentDefinitionLadderShelves.entities.count

Error:

Error: #<TypeError: reference to deleted ComponentDefinition>
C:/Users/gkd/SketchupRubyScripts/ladderShelves2.rb:86:in entities' C:/Users/gkd/SketchupRubyScripts/ladderShelves2.rb:86:in create_component_definition_laddershelves'
C:/Users/gkd/SketchupRubyScripts/ladderShelves2.rb:99:in <module:LadderShelves>' C:/Users/gkd/SketchupRubyScripts/ladderShelves2.rb:5:in <top (required)>'