Fix `nil` returns

[Temikus]

Looks like some errors return a nil which breaks the wait_for routine, see comment in #613

[Temikus]

Hmmm, actually looks like this is a bit more complicated, but collection now behaves incorrectly, i.e. Disks.get(fake) will return a disk object.

Will need to dive a bit deeper on this one.

[github-actions]

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