UnsupportedOperationException in FileViewerServiceImpl

**Affected version : Squash 8.1.1.RELEASE **

Steps to reproduce

Sometimes when trying to open a automated test run report tar file an exceptions is shown in log and an empty file content screen is shown.

Expected result

File content of the listed report tar file should be shown.

Observed behaviour

File content of the listed report tar file is not shown and an exception is been thrown (see attached log).

java.lang.UnsupportedOperationException: null
        at java.base/java.util.ImmutableCollections.uoe(Unknown Source) ~[?:?]
        at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.add(Unknown Source) ~[?:?]
        at org.squashtest.tm.service.internal.display.attachment.FileViewerServiceImpl.processParent(FileViewerServiceImpl.java:184) ~[tm.service-8.1.1.RELEASE.jar!/:?]
...

UnsupportedOperationException.txt