Qt ConfirmTxDialog: make sure dialog is deleted when closed

Same for BlockingWaitingDialog.

related: https://github.com/spesmilo/electrum/issues/3956#issuecomment-1017593613
Note that this change does not solve the "dialog sometimes does not get drawn properly" issue,
just the "dialog sometimes does not get closed properly" issue.

closes: https://github.com/spesmilo/electrum/issues/7816

electrum/gui/qt/confirm_tx_dialog.py

@@ -193,6 +193,7 @@ class ConfirmTxDialog(TxEditor, WindowModalDialog):
     def run(self):
         cancelled = not self.exec_()
         password = self.pw.text() or None
+        self.deleteLater()  # see #3956
         return cancelled, self.is_send, password, self.tx
 
     def on_send(self):

electrum/gui/qt/util.py

@@ -337,15 +337,18 @@ class BlockingWaitingDialog(WindowModalDialog):
         self.message_label = QLabel(message)
         vbox = QVBoxLayout(self)
         vbox.addWidget(self.message_label)
+        self.finished.connect(self.deleteLater)  # see #3956
         # show popup
         self.show()
         # refresh GUI; needed for popup to appear and for message_label to get drawn
         QCoreApplication.processEvents()
         QCoreApplication.processEvents()
-        # block and run given task
-        task()
-        # close popup
-        self.accept()
+        try:
+            # block and run given task
+            task()
+        finally:
+            # close popup
+            self.accept()
 
 
 def line_dialog(parent, title, label, ok_label, default=None):