I believe lightning requests created before https://github.com/spesmilo/electrum/pull/7730
can have an amount of None - ones created after have amount 0 instead.
We could do a wallet db upgrade potentially.
Regardless, the type hint is `get_amount_sat(self) -> Union[int, str, None]`,
so None should be handled. (well, arguably "!" should be handled too...)
```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
File "...\electrum\electrum\gui\qt\request_list.py", line 101, in item_changed
self.parent.show_receive_request(req)
File "...\electrum\electrum\gui\qt\main_window.py", line 1279, in show_receive_request
URI = req.get_bip21_URI(lightning=bip21_lightning)
File "...\electrum\electrum\invoices.py", line 164, in get_bip21_URI
amount = int(self.get_amount_sat())
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'
```
```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
File "...\electrum\electrum\gui\qt\request_list.py", line 101, in item_changed
self.parent.show_receive_request(req)
File "...\electrum\electrum\gui\qt\main_window.py", line 1281, in show_receive_request
can_receive_lightning = self.wallet.lnworker and req.get_amount_sat() <= self.wallet.lnworker.num_sats_can_receive()
TypeError: '<=' not supported between instances of 'NoneType' and 'decimal.Decimal'
```
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