1. Download https://www.iaas.uni-stuttgart.de/publications/INPROC-2014-24-Lego4TOSCA-Composable-Building-Blocks-for-Cloud-Applications.pdf
2. Start JabRef
3. Create a new library
4. Save the library
5. Drag and drop INPROC-2014-24-Lego4TOSCA-Composable-Building-Blocks-for-Cloud-Applications.pdf into the library
6. JabRef shows following dialog:
   

Expected:

1. No dialog appears, because there is only one "real" source of information
2. If there are other cases of PDFs with more columns, an empty column should not be shown

Task

1. Find out where this dialog is called whren dropping a PDF to the main table
2. Adapt code accordingly. Maybe, an existing entry should not be passed at all? Maybe an empty entry needs to be removed.