object not locked by thread before wait()

What steps will reproduce the problem?
1.Open a book (in my case djvu, but it doesn't matter, probably)
2.Collapse the application, or switch to another app, which uses a lot of 
memory (apparently it makes other apps unloaded from the memory)
3.Switch back to eBookDroid

What is the expected output?
I'd expect to continue reading the book I opened.

What do you see instead?
An unexpected error dialog, application is closed, and after restarting you see 
the book list (still can continue reading from the last point, though)

What version of the product are you using?
2.1.12 (keep it up-to-date) 

On what operating system?
Android 4.2.2

What device are you using?
Samsung GT-P5100

Please provide any additional information below.
I always have troubles collapsing my apps, most of them are restarted when 
switched back to. Too little memory, perhaps? Never get errors, though.

Original issue reported on code.google.com by bearmeth...@gmail.com on 25 Sep 2014 at 8:30

Merged into: #853

ngmicrosystems / ebookdroid

object not locked by thread before wait() #870