command-Z clear the content of a file

Steps to reproduce:

1. Open Designer
2. Go to Resources
3. Open a file (whatever the file, a document layout, a result layout, ...)
4. Click in the file (so the editor has the focus)
5. Hit command-Z (I suppose ctrl-z on Windows)

=> The content is all wiped out, you now have a blank page
(DO NOT SAVE, it really is blank, it is not a display issue)

Other way to reproduce:

• Modify the file several time
• Undo, undo, undo... once you reach the document before you modify it, if you undo, the content is wiped out

Expected behavior: When there is nothing to undo while the editor has the focus, nothing should happen