Fix bug.

app/models/seminar_submission_value.rb

@@ -12,7 +12,7 @@ class SeminarSubmissionValue
   before_save :check_key
   before_save :data_proc
   after_save do
-    if (self.seminar_submission_field.markup == "seminar_preferred_session" rescue false)
+    if (self.seminar_submission_field.markup == "seminar_preferred_session" rescue false) && self.seminar_signup_contribute
       val_change = self.val_change
       self.seminar_signup_contribute.update(:preferred_session=>val.to_i)
       self.seminar_signup_contribute.seminar_signup.set_preferred_sessions