kivy: fix open channel with "max" amt

related #6169

E | gui.kivy.uix.dialogs.lightning_open_channel.LightningOpenChannelDialog | Problem opening channel
Traceback (most recent call last):
  File "/home/user/wspace/electrum/electrum/gui/kivy/uix/dialogs/lightning_open_channel.py", line 167, in do_open_channel
    chan, funding_tx = lnworker.open_channel(
  File "/home/user/wspace/electrum/electrum/lnworker.py", line 859, in open_channel
    if funding_sat > LN_MAX_FUNDING_SAT:
TypeError: '>' not supported between instances of 'str' and 'int'

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

@@ -9,6 +9,7 @@ from electrum.util import bh2u
 from electrum.bitcoin import COIN
 import electrum.simple_config as config
 from electrum.logging import Logger
+from electrum.lnutil import ln_dummy_address
 
 from .label_dialog import LabelDialog
 
@@ -164,10 +165,17 @@ class LightningOpenChannelDialog(Factory.Popup, Logger):
         lnworker = self.app.wallet.lnworker
         try:
             funding_tx = lnworker.mktx_for_open_channel(coins=coins, funding_sat=amount)
+        except Exception as e:
+            self.logger.exception("Problem opening channel")
+            self.app.show_error(_('Problem opening channel: ') + '\n' + repr(e))
+            return
+        # read funding_sat from tx; converts '!' to int value
+        funding_sat = funding_tx.output_value_for_address(ln_dummy_address())
+        try:
             chan, funding_tx = lnworker.open_channel(
                 connect_str=conn_str,
                 funding_tx=funding_tx,
-                funding_sat=amount,
+                funding_sat=funding_sat,
                 push_amt_sat=0,
                 password=password)
         except Exception as e: