Animation Editor Bug moving a cel from a transparent layer to background

[GoogleCodeExporter]

What steps will reproduce the problem?
1.Open the program, create a new project.
2.Draw anithing.
3.Create a new layer
4.Press TAB key or go to View > Animation Editor to open the Animation Editor.
5.Move the Background cel to the first layer.
6.Now, move the cel of the first layer to the background layer.
7.Press Ctrl + z, or go to Edit > Undo.
8.Error, but the program do not crash

What is the expected output? What do you see instead?
Well, I think that in this case, undo should do nothing, because when I undo in 
the step 7, what the program do is move the backgroud cel again into the first 
layer, leaving the cel on the background layer empty. And that can not be 
possible.

What version of the product are you using? On what operating system?
0.9.5, Windows 7.

*Please provide any additional information below. If the program crashed on
Windows, you can attach the aseprite-memory.dmp file generated in the same
location where aseprite.exe is.*

Original issue reported on code.google.com by al.fre....@gmail.com on 16 Aug 2013 at 5:58

[GoogleCodeExporter]

Original comment by davidcapello on 25 Nov 2013 at 8:15

• Added labels: UI-Timeline

[GoogleCodeExporter]

Original comment by davidcapello on 10 Apr 2014 at 1:39

• Added labels: Milestone-1.0

[GoogleCodeExporter]

Original comment by davidcapello on 30 Apr 2014 at 2:22

• Changed state: Accepted
• Added labels: Priority-Critical
• Removed labels: Priority-Medium

[GoogleCodeExporter]

Fixed. Commit: 
https://github.com/aseprite/aseprite/commit/09027fdee424c7cff1d9ff5aed22456fe453
2b19

Original comment by davidcapello on 2 May 2014 at 2:30

• Changed title: Animation Editor Bug moving a cel from a transparent layer to background
• Changed state: Fixed

[GoogleCodeExporter]

Original comment by davidcapello on 17 Jul 2014 at 1:03

• Added labels: Component-UI-Timeline
• Removed labels: UI-Timeline