ParaView output with multiple producers for one timestep shows `Cannot display file: undefined`

[EthanS94]

@dhrogers

This database has multiple producers per timestep (1 png and 1 vtm). When loading this cdb into explorer, it shows Cannot display file: undefined for the empty element in each line. The cdb is correct. The viewer is not interpreting it correctly.

Database to reproduce:
test_cdb_geom_and_images.cdb.tar.gz

1. Use ParaView 5.9.0-RC3 built-in viewer to load database into explorer: /Applications/ParaView-5.9.0-RC3.app/Contents/bin/pvpython -m paraview.tpl.cinemasci.server --viewer explorer --data ./test_cdb_geom_and_images.cdb --port 8000
2. Open URL from command line