fix: psbt_nostr: don't allow to save tx without txid

diff --git a/electrum/plugins/psbt_nostr/psbt_nostr.py b/electrum/plugins/psbt_nostr/psbt_nostr.py
index b084c0e..de085bf 100644
--- a/electrum/plugins/psbt_nostr/psbt_nostr.py
+++ b/electrum/plugins/psbt_nostr/psbt_nostr.py
@@ -278,6 +278,7 @@ class CosignerWallet(Logger):
         on_failure: Callable = None,
         on_success: Callable = None
     ) -> None:
+        assert tx.txid(), "Shouldn't allow to save tx without txid"
         try:
             # TODO: adding tx should be handled more gracefully here:
             # 1) don't replace tx with same tx with less signatures
diff --git a/electrum/plugins/psbt_nostr/qml.py b/electrum/plugins/psbt_nostr/qml.py
index 7275fc3..3ad7ec1 100644
--- a/electrum/plugins/psbt_nostr/qml.py
+++ b/electrum/plugins/psbt_nostr/qml.py
@@ -48,7 +48,7 @@ class QReceiveSignalObject(QObject):
         QObject.__init__(self)
         self._plugin = plugin
 
-    cosignerReceivedPsbt = pyqtSignal(str, str, str, str)
+    cosignerReceivedPsbt = pyqtSignal(str, str, str, str, bool)
     sendPsbtFailed = pyqtSignal(str, arguments=['reason'])
     sendPsbtSuccess = pyqtSignal()
 
@@ -138,7 +138,8 @@ class QmlCosignerWallet(EventListener, CosignerWallet):
     def on_event_psbt_nostr_received(self, wallet, pubkey, event_id, tx: 'PartialTransaction', label: str):
         if self.wallet == wallet:
             self.tx = tx
-            self.plugin.so.cosignerReceivedPsbt.emit(pubkey, event_id, tx.serialize(), label)
+            can_be_saved = tx.txid() is not None
+            self.plugin.so.cosignerReceivedPsbt.emit(pubkey, event_id, tx.serialize(), label, can_be_saved)
 
     def close(self):
         super().close()
@@ -173,5 +174,5 @@ class QmlCosignerWallet(EventListener, CosignerWallet):
     def reject_psbt(self, event_id):
         self.mark_pending_event_rcvd(event_id)
 
-    def on_add_fail(self):
-        self.logger.error('failed to add tx to wallet')
+    def on_add_fail(self, error_msg: str):
+        self.logger.error(f'failed to add tx to wallet: {error_msg}')
diff --git a/electrum/plugins/psbt_nostr/qml/PsbtReceiveDialog.qml b/electrum/plugins/psbt_nostr/qml/PsbtReceiveDialog.qml
index 19d1352..286324f 100644
--- a/electrum/plugins/psbt_nostr/qml/PsbtReceiveDialog.qml
+++ b/electrum/plugins/psbt_nostr/qml/PsbtReceiveDialog.qml
@@ -17,6 +17,7 @@ ElDialog {
     }
 
     property string tx_label
+    property bool can_be_saved
     property int choice: PsbtReceiveDialog.Choice.None
 
     // TODO: it might be better to defer popup until no dialogs are shown
@@ -81,6 +82,7 @@ ElDialog {
                 Layout.preferredWidth: 1
                 text: qsTr('Save to Wallet')
                 icon.source: Qt.resolvedUrl('../../../gui/icons/wallet.png')
+                visible: dialog.can_be_saved
                 onClicked: {
                     choice = PsbtReceiveDialog.Choice.Save
                     doAccept()
diff --git a/electrum/plugins/psbt_nostr/qml/main.qml b/electrum/plugins/psbt_nostr/qml/main.qml
index 2c2d5f9..3990738 100644
--- a/electrum/plugins/psbt_nostr/qml/main.qml
+++ b/electrum/plugins/psbt_nostr/qml/main.qml
@@ -7,9 +7,10 @@ import "../../../gui/qml/components/controls"
 Item {
     Connections {
         target: AppController ? AppController.plugin('psbt_nostr') : null
-        function onCosignerReceivedPsbt(pubkey, event, tx, label) {
+        function onCosignerReceivedPsbt(pubkey, event, tx, label, can_be_saved) {
             var dialog = psbtReceiveDialog.createObject(app, {
-                tx_label: label
+                tx_label: label,
+                can_be_saved: can_be_saved
             })
             dialog.accepted.connect(function () {
                 if (dialog.choice == PsbtReceiveDialog.Choice.Open) {
diff --git a/electrum/plugins/psbt_nostr/qt.py b/electrum/plugins/psbt_nostr/qt.py
index dc9f86f..eee3285 100644
--- a/electrum/plugins/psbt_nostr/qt.py
+++ b/electrum/plugins/psbt_nostr/qt.py
@@ -102,7 +102,8 @@ class QtCosignerWallet(EventListener, CosignerWallet):
             self.obj.cosignerReceivedPsbt.emit(*args)  # put on UI thread via signal
 
     def send_to_cosigners(self, tx: Union['Transaction', 'PartialTransaction'], label: str):
-        self.add_transaction_to_wallet(tx, label=label, on_failure=self.on_add_fail)
+        if tx.txid():
+            self.add_transaction_to_wallet(tx, label=label, on_failure=self.on_add_fail)
         self.send_psbt(tx, label)
 
     def do_send(self, messages: List[Tuple[str, dict]], txid: Optional[str] = None):
@@ -120,8 +121,10 @@ class QtCosignerWallet(EventListener, CosignerWallet):
         except Exception as e:
             self.window.show_error(str(e))
             return
-        self.window.show_message(
-            _("Your transaction was sent to your cosigners via Nostr.") + '\n\n' + txid)
+        message = _("Your transaction was sent to your cosigners via Nostr.")
+        if txid:
+            message += '\n\n' + txid
+        self.window.show_message(message)
 
     def on_receive(self, pubkey, event_id, tx, label):
         msg = '<br/>'.join([
@@ -129,13 +132,17 @@ class QtCosignerWallet(EventListener, CosignerWallet):
             _("A transaction was received from your cosigner with label: <br/><big>{}</big><br/>").format(label),
             _("Do you want to open it now?")
         ])
-        result = self.window.show_message(msg, rich_text=True, icon=QMessageBox.Icon.Question, buttons=[
-                QMessageBox.StandardButton.Open,
-                (QPushButton('Discard'), QMessageBox.ButtonRole.DestructiveRole, 100),
-                (QPushButton('Save to wallet'), QMessageBox.ButtonRole.AcceptRole, 101)]
-        )
+        buttons = [
+            QMessageBox.StandardButton.Open,
+            (QPushButton('Discard'), QMessageBox.ButtonRole.DestructiveRole, 100),
+        ]
+        if tx.txid():  # cannot add tx without txid to wallet history (e.g. unsigned legacy tx)
+            buttons.append(
+                (QPushButton('Save to wallet'), QMessageBox.ButtonRole.AcceptRole, 101)  # type: ignore
+            )
+        result = self.window.show_message(msg, rich_text=True, icon=QMessageBox.Icon.Question, buttons=buttons)
         if result == QMessageBox.StandardButton.Open:
-            if label:
+            if label and tx.txid():
                 self.wallet.set_label(tx.txid(), label)
             show_transaction(tx, parent=self.window, prompt_if_unsaved=True, on_closed=partial(self.on_tx_dialog_closed, event_id))
         else: