bug(web): `Error: No matching stub has been registered.`

Not sure what is causing this; 662 errors in last 90 days. The error message could use more context.

Sentry Issue: [KEYMANWEB-COM-2C8](https://keyman.sentry.io/issues/6793690057/?referrer=github_integration)

```
Error: No matching stub has been registered.
at {_ContextManagerBase#2}.prepareKeyboardForActivation (https://s.keyman.com/kmw/engine/18.0.245/src/web/src/engine/main/src/contextManagerBase.ts:315:15)
at wl. (https://s.keyman.com/kmw/engine/18.0.245/src/web/src/engine/main/src/contextManagerBase.ts:240:37)
at Generator.next ()
at None (/kmw/engine/18.0.245/keymanweb.js:1:1396)
at new Promise ()
...
(15 additional frame(s) were not displayed)
```