Fix tags view for missing taggables

When the taggable has been deleted it is nil in this collection
and model_name is not available on NilClass

app/views/alchemy/admin/tags/_tag.html.erb

@@ -2,7 +2,7 @@
   <td class="icon"><%= render_icon(:tag, size: "xl") %></td>
   <td class="name"><%= tag.name %></td>
   <td>
-    <%= tag.taggings.collect(&:taggable).collect { |t| t.class.model_name.human }.uniq.join(', ') %>
+    <%= tag.taggings.collect(&:taggable).compact.map { |t| t.class.model_name.human }.uniq.join(', ') %>
   </td>
   <td class="count"><%= tag.taggings.count %></td>
   <td class="tools">

spec/controllers/alchemy/admin/tags_controller_spec.rb

@@ -10,10 +10,26 @@ module Admin
       before { authorize_user(:as_admin) }
 
       describe "#index" do
+        render_views
+
+        let!(:picture) { create(:alchemy_picture, tag_list: "Foo,Bar") }
+
         it "renders index template" do
           get :index
           expect(response).to be_successful
         end
+
+        context "with taggable missing" do
+          before do
+            picture.thumbs.destroy_all
+            picture.delete
+          end
+
+          it "does not raise error" do
+            get :index
+            expect(response).to be_successful
+          end
+        end
       end
 
       describe "#create" do