Merge pull request #16951 from d-m-u/bz_1540713

Add nil check to serializer for invalid categories
(cherry picked from commit 0e35ec3)

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1542606

app/models/dialog_field_serializer.rb

@@ -23,7 +23,7 @@ def serialize(dialog_field, all_attributes = false)
 
     if dialog_field.type == "DialogFieldTagControl"
       category = Category.find_by(:id => dialog_field.category)
-      dialog_field.options.merge!(:category_name => category.name, :category_description => category.description)
+      dialog_field.options.merge!(:category_name => category.name, :category_description => category.description) if category
     end
     included_attributes(dialog_field.as_json(:methods => [:type, :values]), all_attributes).merge(extra_attributes)
   end