Fix segfault when shutdown during wallet open

If you open a wallet and send a shutdown signal during
that process, the GUI will segfault due to some queued
wallet events happening after the wallet controller
is deleted. This is a minimal fix for those issues.

src/qt/bitcoingui.cpp

@@ -680,6 +680,10 @@ void BitcoinGUI::setWalletController(WalletController* wallet_controller)
 
     GUIUtil::ExceptionSafeConnect(wallet_controller, &WalletController::walletAdded, this, &BitcoinGUI::addWallet);
     connect(wallet_controller, &WalletController::walletRemoved, this, &BitcoinGUI::removeWallet);
+    connect(wallet_controller, &WalletController::destroyed, this, [this] {
+        // wallet_controller gets destroyed manually, but it leaves our member copy dangling
+        m_wallet_controller = nullptr;
+    });
 
     auto activity = new LoadWalletsActivity(m_wallet_controller, this);
     activity->load();
@@ -692,7 +696,7 @@ WalletController* BitcoinGUI::getWalletController()
 
 void BitcoinGUI::addWallet(WalletModel* walletModel)
 {
-    if (!walletFrame) return;
+    if (!walletFrame || !m_wallet_controller) return;
 
     WalletView* wallet_view = new WalletView(walletModel, platformStyle, walletFrame);
     if (!walletFrame->addView(wallet_view)) return;
@@ -742,7 +746,7 @@ void BitcoinGUI::removeWallet(WalletModel* walletModel)
 
 void BitcoinGUI::setCurrentWallet(WalletModel* wallet_model)
 {
-    if (!walletFrame) return;
+    if (!walletFrame || !m_wallet_controller) return;
     walletFrame->setCurrentWallet(wallet_model);
     for (int index = 0; index < m_wallet_selector->count(); ++index) {
         if (m_wallet_selector->itemData(index).value<WalletModel*>() == wallet_model) {

src/qt/sendcoinsdialog.cpp

@@ -600,10 +600,15 @@ SendCoinsEntry *SendCoinsDialog::addEntry()
     entry->clear();
     entry->setFocus();
     ui->scrollAreaWidgetContents->resize(ui->scrollAreaWidgetContents->sizeHint());
-    qApp->processEvents();
+
-    QScrollBar* bar = ui->scrollArea->verticalScrollBar();
+    // Scroll to the newly added entry on a QueuedConnection because Qt doesn't
-    if(bar)
+    // adjust the scroll area and scrollbar immediately when the widget is added.
-        bar->setSliderPosition(bar->maximum());
+    // Invoking on a DirectConnection will only scroll to the second-to-last entry.
+    QMetaObject::invokeMethod(ui->scrollArea, [this] {
+        if (ui->scrollArea->verticalScrollBar()) {
+            ui->scrollArea->verticalScrollBar()->setValue(ui->scrollArea->verticalScrollBar()->maximum());
+        }
+    }, Qt::QueuedConnection);
 
     updateTabsAndLabels();
     return entry;

src/qt/test/wallettests.cpp

@@ -212,6 +212,8 @@ void TestGUI(interfaces::Node& node)
     QCOMPARE(transactionTableModel->rowCount({}), 105);
     uint256 txid1 = SendCoins(*wallet.get(), sendCoinsDialog, PKHash(), 5 * COIN, /*rbf=*/false);
     uint256 txid2 = SendCoins(*wallet.get(), sendCoinsDialog, PKHash(), 10 * COIN, /*rbf=*/true);
+    // Transaction table model updates on a QueuedConnection, so process events to ensure it's updated.
+    qApp->processEvents();
     QCOMPARE(transactionTableModel->rowCount({}), 107);
     QVERIFY(FindTx(*transactionTableModel, txid1).isValid());
     QVERIFY(FindTx(*transactionTableModel, txid2).isValid());