do not listed to files if project is already closed + don't forget to add lesson to the course

python/educational/interactive-learning/src/com/jetbrains/edu/learning/StudyProjectComponent.java

@@ -233,6 +233,7 @@ public class StudyProjectComponent implements ProjectComponent {
         }
       }
     }
+    myListener = null;
   }
 
   @Override
@@ -285,6 +286,7 @@ public class StudyProjectComponent implements ProjectComponent {
   private class FileCreatedByUserListener extends VirtualFileAdapter {
     @Override
     public void fileCreated(@NotNull VirtualFileEvent event) {
+      if (myProject.isDisposed()) return;
       final VirtualFile createdFile = event.getFile();
       final VirtualFile taskDir = createdFile.getParent();
       final Course course = StudyTaskManager.getInstance(myProject).getCourse();

python/educational/src/com/jetbrains/edu/courseFormat/Course.java

@@ -4,6 +4,7 @@ import com.intellij.lang.Language;
 import org.jetbrains.annotations.NotNull;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 public class Course {
@@ -17,7 +18,11 @@ public class Course {
   private String myLanguage;
 
   public List<Lesson> getLessons() {
-    return lessons;
+    return Collections.unmodifiableList(lessons);
+  }
+
+  public void addLesson(@NotNull final Lesson lesson) {
+    lessons.add(lesson);
   }
 
   public String getAuthor() {