unnamed fields using `contents` generate compiler errors

[Ithamar]

when I use something like - contents: [ 1, 2, 3 ] in a ksy file, since the 0.9.0 release, I get the following error: unable to access 'unnamed_0' in test_ksy context.

This used to work fine before the 0.9.0 release and is easily reproducable with https://ide.kaitai.io.