Redacting a reaction on an already redacted event makes sync blow up

[ptman]

Describe the bug
Error is: Sync failed because of TypeError: target.annotations is undefined
I get the on the top on green background. I can share logs privately.

Desktop (please complete the following information):

• OS: Linux
• Browser Firefox
• Version 91.0.2
• Hydrogen 0.2.8 (and 0.2.5 had the same problem)

[bwindels]

Notes from having looked at ptman's logs:

so first the message ($1his...) is redacted
then the reaction to that message ($VRF...)
and it sees that $VRF was a reaction and tries to recalculate the remaining reactions on the target of the reaction ($1his ...) and that fails because the reactions there have already been removed during the prior redaction