Reject supporter codes if they're already used by another email

app/models/datatable_supporters.rb

@@ -4,13 +4,23 @@ class DatatableSupporters < Datatable
     items.each do |i|
       item = []
       if i.name.blank?
-        item << i.registration.person.public_name
+        if !i.registration.nil? && !i.registration.person.nil?
+          item << i.registration.person.public_name
+        else
+          item << "Unknown"
+        end
+
       else
         item << i.name
       end
 
       if i.email.blank?
-        item << i.registration.person.email
+        if !i.registration.nil? && !i.registration.person.nil?
+          item << i.registration.person.email
+        else
+          item << "Unknown"
+        end
+
       else
         item << i.email
       end