gui: Unregister wallet notifications before unloading wallets

This change was originally part of both bitcoin/bitcoin#10102 and
bitcoin/bitcoin#19101 and is required for both because it avoids the IPC
wallet implementation in bitcoin/bitcoin#10102 and the WalletContext
implementation in bitcoin/bitcoin#19101 needing to deal with
notification objects that have stale pointers to deleted wallets.

src/qt/bitcoin.cpp

@@ -352,6 +352,17 @@ void BitcoinApplication::requestShutdown()
     window->setClientModel(nullptr);
     pollShutdownTimer->stop();
 
+#ifdef ENABLE_WALLET
+    // Delete wallet controller here manually, instead of relying on Qt object
+    // tracking (https://doc.qt.io/qt-5/objecttrees.html). This makes sure
+    // walletmodel m_handle_* notification handlers are deleted before wallets
+    // are unloaded, which can simplify wallet implementations. It also avoids
+    // these notifications having to be handled while GUI objects are being
+    // destroyed, making GUI code less fragile as well.
+    delete m_wallet_controller;
+    m_wallet_controller = nullptr;
+#endif // ENABLE_WALLET
+
     delete clientModel;
     clientModel = nullptr;