[BUG] Cancelling conversion clears your list of selected PDF's to convert

**Describe the bug**
When canceling a conversion of one or more PDF's after clicking convert, the chosen PDF's is emptied.

**To Reproduce**
Steps to reproduce the behavior:
1. Add a PDF to be converted via the 'Choose file(s)' button.
2. Click on the 'Convert!' button.
3. Cancel the dialog box.
4. See that no PDF's are available to convert and need to be added back.

**Expected behavior**
Once cancel is clicked or the dialog box closed, the list of PDF's should remain selected.

Start by running the WPF application and reproduce the issue with a selected PDF: click “Convert!” and cancel or close the dialog. Trace the conversion flow to find where the selected PDF list is cleared, then verify that the selection remains available for another conversion after cancellation.

Written by the indexing model from the issue text.