Fix null reference error

Author: rastating

lib/wpxf/utility/body_builder.rb

@@ -71,6 +71,7 @@ def create
       private
 
       def _cleanup_temp_files(fields)
+        return if fields.nil?
         fields.each { |_k, v| v.close if v.is_a?(File) }
         FileUtils.rm_rf @temp_dir.to_s
       end