EDU-537 Empty task text with opened task files

python/educational-core/student/src/com/jetbrains/edu/learning/StudyBasePluginConfigurator.java

@@ -56,6 +56,11 @@ public abstract class StudyBasePluginConfigurator implements StudyPluginConfigur
 
       @Override
       public void fileClosed(@NotNull FileEditorManager source, @NotNull VirtualFile file) {
+        for (VirtualFile openedFile : source.getOpenFiles()) {
+          if (StudyUtils.getTaskFile(project, openedFile) != null) {
+            return;
+          }
+        }
         toolWindow.setEmptyText(project);
       }