fix(web): crash on focus before init

Fixes the following crash noted when testing on keymanweb.com, focusing
during the load process on a slow network.

```
helpers.ts:111 Uncaught TypeError: Cannot read properties of null (reading 'vkbd')
    at g.pageFocusHandler (keymanweb.ts:125:44)
    at sentryWrapped (helpers.ts:87:17)
```