Merge #360: Unregister wallet notifications before unloading wallets

93cc53a gui: Unregister wallet notifications before unloading wallets (Russell Yanofsky)

Pull request description:

  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.

ACKs for top commit:
  promag:
    Code review ACK 93cc53a.
  hebasto:
    ACK 93cc53a

Tree-SHA512: 805f50a493291ad0f7c48725fbc5058d58ebbdb0770befd51d8aa241209a13f8a46f5982481336ab8338cdc83e9017668089a71deccf1587308e841cf8697825

Author: hebasto

src/qt/bitcoin.cpp

@@ -343,6 +343,17 @@ 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;