fix bug

app/controllers/exercises_controller.rb

@@ -1006,6 +1006,7 @@ class ExercisesController < ApplicationController
   def start_answer
     ActiveRecord::Base.transaction do
       begin
+        @exercise_user_current = @exercise.exercise_users.exercise_commit_users(@exercise_current_user_id)&.first
         if @exercise_user_current.blank?
           if @user_course_identity > Course::ASSISTANT_PROFESSOR  #当为老师的时候，不创建exercise_user表，理论上老师是不能进入答题的
             exercise_user_params = {