Unable to set checkpoint

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use menu to "set checkpoint".
2. Make some changes.
3. Use menu to "undo to checkpoint" -- option is grayed out, does not revert 
when used.

What is the expected output? What do you see instead?
One time, out of many attempts, I got the "set checkpoint" to take, and "undo 
to checkpoint" worked and was not greyed out. I have no idea what I did 
differently that time.

What version of the product are you using? On what operating system?
OpenSudoku 1.1.5, Android 2.2.2, 

Please provide any additional information below.
Very newbie user, could be missing something obvious (or not obvious).

Original issue reported on code.google.com by [email protected] on 25 Aug 2011 at 2:23

[GoogleCodeExporter]

Most of the time undo to checkpoint is disabled on my system. I don't know why? 
And I miss it.

Original comment by [email protected] on 6 Mar 2012 at 10:48

[GoogleCodeExporter]

The issue happens whenever I use the hardware back key on my phone. If I 
instead use the hardware home button, there's no problem. 

There's a behavioural difference - the back key gets you to the puzzles list, 
while the home key gets you outside opensudoku altogether. 

Original comment by [email protected] on 24 Apr 2014 at 3:52