Export is crashing after canceling pull when empty submissions are created - Java 8

[mmarciniak90]

Software versions

Briefcase master
Java 8

Problem description

Export is crashing after canceling pull when empty submissions are created

Steps to reproduce the problem

1. Start a pull operation and cancel it.
2. Verify that the last submission started is empty
3. Go to the export tab -> list is empty, the issue is reported Forms aren’t immediately showing up in push or export after a canceled pull - only with Java 8 #888
4. Go to the Settings tab and click on the Reload forms from storage location button
5. Go to the export tab
6. Select form with empty submission and export it
7. Export stops with status: Export has started -> it looks like a crash

Workaround

Reopened and reload storage directory cause that export is possible with Java 8.

Expected behavior

Export is finished

Notes

Related to #884