Check for existence of submissions before accessing them

app/views/external_users/statistics.html.slim

@@ -1,8 +1,8 @@
 h1 = t('.title')
 
 - submissions = Submission.where(user: @user).in_study_group_of(current_user)
-- exercises = Exercise.where(id: submissions.joins(:exercise).group(:exercise_id).select(:exercise_id).distinct)
-- if !policy(exercises.reject(&:nil?).first).detailed_statistics?
+- exercises = Exercise.where(id: submissions.joins(:exercise).group(:exercise_id).select(:exercise_id).distinct).reject(&:nil?)
+- if submissions.any? && !policy(exercises.first).detailed_statistics?
   - submissions = submissions.final
 
 - if submissions.any?