Previously, the ServiceWorker assumed that a client found in
`clientVersionMap` would exist (i.e. it could be retrieved via
`clients.get()`). However, if a browser tab had been closed, the
corresponding client (while present in `clientVersionMap`, which is only
updated on ServiceWorker initialization) would not be retrievable via
`clients.get()`.
This commit fixes it by checking whether the client exists before trying
to notify it about an unrecoverable state.
PR Close#42736
cb2ca9a66e