Ensure content exists before adding it

lib/custom_wizard/builder.rb

@@ -257,7 +257,7 @@ class CustomWizard::Builder
         }
       ).perform
                         
-      if content[:type] == 'association'
+      if content && content[:type] == 'association'
         content[:result] = content[:result].map do |item|
           { 
             id: item[:key],