Guard against calling #thumb on a collection object

lib/dor/models/concerns/publishable.rb

@@ -8,7 +8,7 @@ module Publishable
     # Compute the thumbnail for this object following the rules at https://consul.stanford.edu/display/chimera/The+Rules+of+Thumb
     # @return [String] the computed thumb filename, with the druid prefix and a slash in front of it, e.g. oo000oo0001/filenamewith space.jp2
     def thumb
-       return if contentMetadata.nil?
+       return unless respond_to?(:contentMetadata) && !contentMetadata.nil?
        cm = contentMetadata.ng_xml
        mime_type_finder = "@mimetype='image/jp2' or @mimeType='image/jp2'" # allow the mimetype attribute to be lower or camelcase when searching to make it more robust
        thumb_image=nil

spec/models/concerns/publishable_spec.rb

@@ -38,6 +38,7 @@ class ItemizableItem < ActiveFedora::Base
   before :each do
 
     @item = instantiate_fixture('druid:ab123cd4567', PublishableItem)
+    @collection = instantiate_fixture('druid:ab123cd4567', Dor::Collection)
     @apo  = instantiate_fixture('druid:fg890hi1234', Dor::AdminPolicyObject)
     allow(@item).to receive(:admin_policy_object).and_return(@apo)
     @mods = <<-EOXML
@@ -100,6 +101,11 @@ class ItemizableItem < ActiveFedora::Base
   end
 
   describe '#thumb' do
+    it 'should return nil if there is no contentMetadata datastream' do
+      expect(@collection.thumb).to be_nil
+      expect(@collection.thumb_url).to be_nil
+    end
+
     it 'should return nil if there is no contentMetadata' do
       expect(@item.thumb).to be_nil
       expect(@item.thumb_url).to be_nil