exception when clicking "replace" under DNS's "this proof has failed" tooltip

when i click "replace" - in the tooltip i see when clicking the red "dns" on my profile - nothing happens and i get the following exception:

```
VM58 sitewide-js.js:96 Uncaught TypeError: window.SigModalUis[link_data.sig_gen] is not a constructor
at SigModalLauncher.show_modal (VM58 sitewide-js.js:96)
at HTMLAnchorElement. (VM58 sitewide-js.js:96)
at HTMLBodyElement.dispatch (VM58 sitewide-js.js:16)
at HTMLBodyElement.elemData.handle (VM58 sitewide-js.js:15)
SigModalLauncher.show_modal @ VM58 sitewide-js.js:96
(anonymous) @ VM58 sitewide-js.js:96
dispatch @ VM58 sitewide-js.js:16
elemData.handle @ VM58 sitewide-js.js:15
```

Start at SigModalLauncher.show_modal in sitewide-js.js, using the reported TypeError and sig_gen lookup as the entry point. Reproduce by opening the red DNS indicator on a profile, opening the “this proof has failed” tooltip, and clicking “replace”; done when that action opens the replacement modal without the constructor exception.

Written by the indexing model from the issue text.