Diffeditor is "Waiting for data" from deleted file

1. Go to "Tools" -> "Diff..." and select two files.
   Creator will show the files' differences.
2. Delete one of the files.
3. Click "Reload Diff"
   The diff editor will display ""Waiting for data..." and that's it.

Instead of waiting forever, the diff editor should inform the user which of the files is missing.