Fixes exception occuring when `wallet.lnworker` is `None` and the user
clicks on `New Channel` in the channels list tab. The `New Channel`
button is enabled when the wallet *can* have lightning, not the wallet
actually having lightning enabled.
With this patch the `init_lightning_dialog` will show up if the user
clicks on `New Channel` but lightning is not yet enabled.
I noticed this by restoring from a funded vpub and then
loading the keystore from seed afterwards.
1b325394ba