kivy: fix some bugs when paying 'max'

fixes: #6164

electrum/gui/kivy/main_window.py

@@ -916,7 +916,7 @@ class ElectrumWindow(App):
             return ''
         addr = None
         if self.send_screen:
-            addr = str(self.send_screen.screen.address)
+            addr = str(self.send_screen.address)
         if not addr:
             addr = self.wallet.dummy_address()
         outputs = [PartialTxOutput.from_address_and_value(addr, '!')]
@@ -939,7 +939,11 @@ class ElectrumWindow(App):
     def format_amount(self, x, is_diff=False, whitespaces=False):
         return format_satoshis(x, 0, self.decimal_point(), is_diff=is_diff, whitespaces=whitespaces)
 
-    def format_amount_and_units(self, x):
+    def format_amount_and_units(self, x) -> str:
+        if x is None:
+            return 'none'
+        if x == '!':
+            return 'max'
         return format_satoshis_plain(x, self.decimal_point()) + ' ' + self.base_unit
 
     def format_fee_rate(self, fee_rate):

electrum/gui/kivy/uix/dialogs/invoice_dialog.py

@@ -17,7 +17,7 @@ if TYPE_CHECKING:
 Builder.load_string('''
 <InvoiceDialog@Popup>
     id: popup
-    amount: 0
+    amount: None
     title: ''
     data: ''
     description:''

electrum/gui/kivy/uix/screens.py

@@ -348,7 +348,6 @@ class SendScreen(CScreen):
     def _do_pay_onchain(self, invoice, rbf):
         # make unsigned transaction
         outputs = invoice['outputs']  # type: List[PartialTxOutput]
-        amount = sum(map(lambda x: x.value, outputs))
         coins = self.app.wallet.get_spendable_coins(None)
         try:
             tx = self.app.wallet.make_unsigned_transaction(coins=coins, outputs=outputs)
@@ -362,6 +361,7 @@ class SendScreen(CScreen):
         if rbf:
             tx.set_rbf(True)
         fee = tx.get_fee()
+        amount = sum(map(lambda x: x.value, outputs)) if '!' not in [x.value for x in outputs] else tx.output_value()
         msg = [
             _("Amount to be sent") + ": " + self.app.format_amount_and_units(amount),
             _("Mining fee") + ": " + self.app.format_amount_and_units(fee),