When the user has already loaded a wallet in QML and tries to restore a
new wallet `WCHaveSeed.qml` incorrectly shows a `Finish` button instead
of a `Next` button and raises a KeyError if the user clicks on Finish
instead of resolving the passphrase input view (regression from
https://github.com/spesmilo/electrum/pull/10016).
This happens because `last` of `have_seed` depends on
`NewWalletWizard.is_single_password()` and NewWalletWizard.wants_ext(wizard_data).
`is_single_password()` is true if a wallet is already loaded,
while `wants_ext(wizard_data)` is false as `wants_ext()` depends on `seed_extend: True` in
`wizard_data` which only gets set after `apply()` of `WCHaveSeed` gets
called, however the evaluation of `WCHaveSeed` being the last view
happens before the view is shown.
By calling `checkIsLast()` in the validation timer of `WCHaveSeed` the
`last` property gets set again after `apply()` has been called, so
the view is guaranteed to correctly show either the `Finish` or `Next` button
after a seed has been entered.
e6f73522cb