fix bug

app/templates/exercise_export/exercise_user.html.erb

@@ -249,8 +249,8 @@
                     <tbody>
                     <% if @games.size > 0 %>
                       <% @games.each_with_index do |game, index| %>
-                        <% user_score = q.exercise_shixun_answers.where(exercise_shixun_challenge_id:game.challenge.id,user_id: @ex_user_user.id) %>
                         <% game_score = q.exercise_shixun_challenges.where(challenge_id:game.challenge.id) %>
+                        <% user_score = q.exercise_shixun_answers.where(exercise_shixun_challenge_id: game_score&.id,user_id: @ex_user_user.id) %>
                         <tr>
                           <td><%= index + 1 %></td>
                           <td style="text-align: left;">