[composer-autosave] Use-after-free during snapshot save to file

It could happen that the snapshot save got interrupted by other UI
code, one of them being composer close, which causes the composer's
dispose() execution, even it has more than one ref_count. The dispose()
frees the content widgets, which results in use-after-free when
the snapshot save continues its execution. The interrupt was possible
also due to several asynchronous calls being involved during the save.

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1538865