qt: NewChannelDialog: do some pre-validation for Ok button

Do some validation if the user input in `NewChannelDialog` before
allowing to click the `Ok` button. This is done to make the UI a bit
more intuitive and prevent issues like #10053 in which the user was able
to click `Ok` with a empty trampoline node list which raised an
`IndexError` (probably on testnet4).

electrum/gui/qt/new_channel_dialog.py

@@ -8,6 +8,7 @@ from electrum.lnutil import MIN_FUNDING_SAT
 from electrum.lnworker import hardcoded_trampoline_nodes
 from electrum.util import NotEnoughFunds, NoDynamicFeeEstimates
 from electrum.fee_policy import FeePolicy
+from electrum.lntransport import extract_nodeid, ConnStringFormatError
 
 from .util import (WindowModalDialog, Buttons, OkButton, CancelButton,
                    EnterButton, WWLabel, char_width_in_lineedit)
@@ -43,14 +44,18 @@ class NewChannelDialog(WindowModalDialog):
             vbox.addWidget(QLabel(_('Enter Remote Node ID or connection string or invoice')))
             self.remote_nodeid = QLineEdit()
             self.remote_nodeid.setMinimumWidth(700)
+            self.remote_nodeid.textChanged.connect(self.maybe_enable_ok_button)
             self.suggest_button = QPushButton(self, text=_('Suggest Peer'))
             self.suggest_button.clicked.connect(self.on_suggest)
         else:
             self.trampoline_combo = QComboBox()
             self.trampoline_combo.addItems(self.trampoline_names)
+            # index 1 is "Electrum trampoline" on mainnet, this defaults to -1 if 1 is not available
             self.trampoline_combo.setCurrentIndex(1)
+            self.trampoline_combo.currentIndexChanged.connect(self.maybe_enable_ok_button)
         self.amount_e = BTCAmountEdit(self.window.get_decimal_point)
         self.amount_e.setAmount(amount_sat)
+        self.amount_e.textChanged.connect(self.maybe_enable_ok_button)
 
         btn_width = 10 * char_width_in_lineedit()
         self.min_button = EnterButton(_("Min"), self.spend_min)
@@ -83,9 +88,26 @@ class NewChannelDialog(WindowModalDialog):
 
         vbox.addLayout(h)
         vbox.addStretch()
-        ok_button = OkButton(self)
-        ok_button.setDefault(True)
-        vbox.addLayout(Buttons(CancelButton(self), ok_button))
+        self.ok_button = OkButton(self)
+        self.ok_button.setDefault(True)
+        self.maybe_enable_ok_button()
+        vbox.addLayout(Buttons(CancelButton(self), self.ok_button))
+
+    def maybe_enable_ok_button(self):
+        enable = True
+        if self.network.channel_db:
+            try:
+                extract_nodeid(str(self.remote_nodeid.text()).strip())
+            except ConnStringFormatError:
+                enable = False
+        else:
+            try:
+                self.trampoline_names[self.trampoline_combo.currentIndex()]
+            except IndexError:
+                enable = False
+        if not self.amount_e.get_amount():
+            enable = False
+        self.ok_button.setEnabled(enable)
 
     def on_suggest(self):
         self.network.start_gossip()