Added non existent teacher validation to vote

docker/backend/src/mw/schema/mutation.cr

@@ -111,6 +111,10 @@ module MW
       def create_vote(context : Context, input : VoteCreateInput) : Vote
         context.role! UserRole::Student
 
+        if input.teacher_ids.any? { |id| Db::Teacher.find(id).nil? }
+          raise "Teachers not found"
+        end
+
         student = context.external.not_nil!.as(Db::Student)
         vote = Db::Vote.create!(student_id: student.id)
         Db::TeacherVote.import(input.teacher_ids.map_with_index { |id, i| Db::TeacherVote.new(vote_id: vote.id, teacher_id: id.to_i64, priority: i) })