Backup system doesn't report error messages via console in headless mode

after a gradle shadowJar build, the following command:
java -jar build/libs/jalview-all-DEVELOPMENT-j11.jar -props ~/Documents/Jalview/ReleaseTests/2.11.1.0/JAL-3386_jvprops -headless -open examples/uniref50.fa - png ~/Documents/Jalview/ReleaseTests/2.11.1.0/JAL-3386_pngoutput

Yielded a stack trace (below) from the output file backup system (JAL-3141). It looks like the backup system isn't honouring the 'nogui' flag and trying to pop open a dialog. It also isn't clear from the log why it "Could not get canonical path for file 'png'" !

Running jalview.bin.Jalview with -Xmx15461882266
Java version: 12
x86_64 Mac OS X 10.13.6
Jalview Version: DEVELOPMENT (Source git-commit:325d9af46 [develop])
CMD [-props /Users/jprocter/Documents/Jalview/ReleaseTests/2.11.1.0/JAL-3386_jvprops] executed successfully!
DEBUG - Jalview Debugging Output Follows.
Quaqua LaF not available on this plaform. Using VAqua(4).
See https://issues.jalview.org/browse/JAL-2976
Ignoring SO:0001957 as obsolete and duplicated by SO:0001979
Ignoring SO:0000437 as obsolete and duplicated by SO:0000800
Ignoring SO:0000554 as obsolete and duplicated by SO:0000801
Ignoring SO:0000767 as obsolete and duplicated by SO:0000179
Ignoring SO:0000058 as obsolete and duplicated by SO:0000803
Ignoring SO:0001052 as obsolete and duplicated by SO:0001649
Ignoring SO:0000052 as obsolete and duplicated by SO:0000802
Ignoring SO:0001053 as obsolete and duplicated by SO:0001648
Loaded Sequence Ontology from so-xp-simple.obo.zip (212ms)
CMD [-open examples/uniref50.fa] executed successfully!
File format identified as Fasta
Feb 11, 2020 9:36:13 AM jalview.util.MessageManager <clinit>
INFO: Getting messages for lang: en_GB
Could not get canonical path for file 'png'
Exception in thread "main" java.awt.HeadlessException
at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:197)
at java.desktop/java.awt.Window.<init>(Window.java:538)
at java.desktop/java.awt.Frame.<init>(Frame.java:423)
at java.desktop/java.awt.Frame.<init>(Frame.java:388)
at java.desktop/javax.swing.SwingUtilities$SharedOwnerFrame.<init>(SwingUtilities.java:1919)
at java.desktop/javax.swing.SwingUtilities.getSharedOwnerFrame(SwingUtilities.java:1995)
at java.desktop/javax.swing.JOptionPane.getRootFrame(JOptionPane.java:1689)
at java.desktop/javax.swing.JOptionPane.showOptionDialog(JOptionPane.java:868)
at java.desktop/javax.swing.JOptionPane.showConfirmDialog(JOptionPane.java:799)
at java.desktop/javax.swing.JOptionPane.showConfirmDialog(JOptionPane.java:761)
at jalview.gui.JvOptionPane.showConfirmDialog(JvOptionPane.java:64)
at jalview.io.BackupFiles.rollBackupsAndRenameTempFile(BackupFiles.java:664)
at jalview.gui.AlignFrame.saveAlignment(AlignFrame.java:1221)
at jalview.bin.Jalview.doMain(Jalview.java:655)
at jalview.bin.Jalview.main(Jalview.java:191)