kivy: fix paying onchain invoices

when pasting a new invoice and paying it
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/gui/kivy/uix/screens.py", line 358, in _do_send_onchain
    tx = self.app.wallet.make_unsigned_transaction(coins, outputs, None)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 849, in make_unsigned_transaction
    if o.type == TYPE_ADDRESS:
AttributeError: 'tuple' object has no attribute 'type'

when loading back a saved invoice
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/gui/kivy/uix/screens.py", line 358, in _do_send_onchain
    tx = self.app.wallet.make_unsigned_transaction(coins, outputs, None)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 849, in make_unsigned_transaction
    if o.type == TYPE_ADDRESS:
AttributeError: 'list' object has no attribute 'type'

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

@@ -1,3 +1,5 @@
+from typing import TYPE_CHECKING
+
 from kivy.factory import Factory
 from kivy.lang import Builder
 from kivy.core.clipboard import Clipboard
@@ -7,6 +9,9 @@ from kivy.clock import Clock
 from electrum.gui.kivy.i18n import _
 from electrum.util import pr_tooltips
 
+if TYPE_CHECKING:
+    from electrum.gui.kivy.main_window import ElectrumWindow
+
 
 Builder.load_string('''
 <InvoiceDialog@Popup>
@@ -58,7 +63,7 @@ class InvoiceDialog(Factory.Popup):
 
     def __init__(self, title, data, key):
         Factory.Popup.__init__(self)
-        self.app = App.get_running_app()
+        self.app = App.get_running_app()  # type: ElectrumWindow
         self.title = title
         self.data = data
         self.key = key