message: Fixed a bug delete empty folder of mediamanager
diff: @@ -115,7 +115,9 @@ courseSchema.pre('remove', async function () {
  const localCourse = <ICourseModel><any>this;
  try {
  const dic = await Directory.findById(localCourse.media);
  + if (dic) {
  await dic.remove();
  + }
  for (const lec of localCourse.lectures) {
  const lecDoc = await Lecture.findById(lec);
  await lecDoc.remove();
  