I was exploring the new widget interface and clicked an existing widget in my footer, which was a legacy block. On clicking preview, I got the following error.

My recommendation I think would be to have preview working, but I am not aware of why it didn't work in this case. Perhaps something didn't activate with the legacy block?
Potentially some more details here, merging: https://github.com/WordPress/gutenberg/issues/25762
Removing needs testing as seems @paaljoachim confirms in the other issue this is also case for him.
This works for me running latest master.

@karmatosed @paaljoachim: Could you check again? Are there any errors in the DevTools console?
I am still seeing the error @noisysocks Robert.
Another test using Twenty Twenty theme with Gutenberg plugin 9.1. This time using a live online test site.

DevTools console:

Looks like this was fixed by https://github.com/WordPress/gutenberg/pull/25792 but let me know if you're still encountering the error and we can re-open.
I am giving a confirmation that the error is now gone. Previewing works in 9.1.1..
