From 2307d6e64ce9d7253d419439cb2410a568528de2 Mon Sep 17 00:00:00 2001
From: Ekaterina Tuzova <Ekaterina.Tuzova@jetbrains.com>
Date: Fri, 31 Mar 2017 17:25:42 +0300
Subject: [PATCH] fix navigation to next/previous task for empty task list

---
 .../learning/navigation/StudyNavigator.java   | 20 +++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/python/educational-core/src/com/jetbrains/edu/learning/navigation/StudyNavigator.java b/python/educational-core/src/com/jetbrains/edu/learning/navigation/StudyNavigator.java
index 91b8a0c7aaf8..85b709a9dc63 100644
--- a/python/educational-core/src/com/jetbrains/edu/learning/navigation/StudyNavigator.java
+++ b/python/educational-core/src/com/jetbrains/edu/learning/navigation/StudyNavigator.java
@@ -41,7 +41,15 @@ public class StudyNavigator {
     if (nextLesson == null) {
       return null;
     }
-    return StudyUtils.getFirst(nextLesson.getTaskList());
+    List<Task> nextLessonTaskList = nextLesson.getTaskList();
+    while (nextLessonTaskList.isEmpty()) {
+      nextLesson = nextLesson(nextLesson);
+      if (nextLesson == null) {
+        return null;
+      }
+      nextLessonTaskList = nextLesson.getTaskList();
+    }
+    return StudyUtils.getFirst(nextLessonTaskList);
   }
 
   public static Task previousTask(@NotNull final Task task) {
@@ -55,7 +63,15 @@ public class StudyNavigator {
       return null;
     }
     //getting last task in previous lesson
-    return prevLesson.getTaskList().get(prevLesson.getTaskList().size() - 1);
+    List<Task> prevLessonTaskList = prevLesson.getTaskList();
+    while (prevLessonTaskList.isEmpty()) {
+      prevLesson = previousLesson(prevLesson);
+      if (prevLesson == null) {
+        return null;
+      }
+      prevLessonTaskList = prevLesson.getTaskList();
+    }
+    return prevLessonTaskList.get(prevLessonTaskList.size() - 1);
   }
 
   public static Lesson nextLesson(@NotNull final Lesson lesson) {