Order of lessons

[humitos]

Step to reproduce it:

1. Open Typing Turtle
2. Go to Edit Lessons screen
3. Click on "+" sign to add a new lesson
4. Write a title
5. Click in "Go Back"
6. Click on "Stop" to close the Activity
7. Open Typing Turtle again

Issue:

I get this error:

Traceback (most recent call last):
  File "/home/humitos/Activities/TypingTurtle.activity/mainscreen.py", line 252, in prev_lesson_clicked_cb
    self.show_lesson(self.lesson_index-1)
  File "/home/humitos/Activities/TypingTurtle.activity/mainscreen.py", line 232, in show_lesson
    next_index = self.get_next_lesson()
  File "/home/humitos/Activities/TypingTurtle.activity/mainscreen.py", line 150, in get_next_lesson
    if self.lessons[i]['order'] >= 0 and \
KeyError: 'order'

Expected issue:

Get the correct order of lessons.

NOTE: this happens because the order of the lessons is saved when the "Go Back" button is pressed in the Main screen of Edit Lessons screen, so if we close the activity without pressing that button the order is not saved