Cancel wallet balance timer when shutdown requested

This doesn't fix any current problem, but it makes balance checking code less
fragile, and prevents use-after free travis error in next commit:
https://travis-ci.org/github/bitcoin/bitcoin/jobs/675367629#L4240

src/qt/walletmodel.cpp

@@ -38,14 +38,15 @@
 WalletModel::WalletModel(std::unique_ptr<interfaces::Wallet> wallet, ClientModel& client_model, const PlatformStyle *platformStyle, QObject *parent) :
     QObject(parent),
     m_wallet(std::move(wallet)),
-    m_client_model(client_model),
+    m_client_model(&client_model),
     m_node(client_model.node()),
     optionsModel(client_model.getOptionsModel()),
     addressTableModel(nullptr),
     transactionTableModel(nullptr),
     recentRequestsTableModel(nullptr),
     cachedEncryptionStatus(Unencrypted),
-    cachedNumBlocks(0)
+    cachedNumBlocks(0),
+    timer(new QTimer(this))
 {
     fHaveWatchOnly = m_wallet->haveWatchOnly();
     addressTableModel = new AddressTableModel(this);
@@ -63,11 +64,16 @@ WalletModel::~WalletModel()
 void WalletModel::startPollBalance()
 {
     // This timer will be fired repeatedly to update the balance
-    QTimer* timer = new QTimer(this);
     connect(timer, &QTimer::timeout, this, &WalletModel::pollBalanceChanged);
     timer->start(MODEL_UPDATE_DELAY);
 }
 
+void WalletModel::setClientModel(ClientModel* client_model)
+{
+    m_client_model = client_model;
+    if (!m_client_model) timer->stop();
+}
+
 void WalletModel::updateStatus()
 {
     EncryptionStatus newEncryptionStatus = getEncryptionStatus();

src/qt/walletmodel.h

@@ -144,6 +144,7 @@ public:
 
     interfaces::Node& node() const { return m_node; }
     interfaces::Wallet& wallet() const { return *m_wallet; }
+    void setClientModel(ClientModel* client_model);
     int getNumBlocks() const { return cachedNumBlocks; }
 
     QString getWalletName() const;
@@ -161,7 +162,7 @@ private:
     std::unique_ptr<interfaces::Handler> m_handler_show_progress;
     std::unique_ptr<interfaces::Handler> m_handler_watch_only_changed;
     std::unique_ptr<interfaces::Handler> m_handler_can_get_addrs_changed;
-    ClientModel& m_client_model;
+    ClientModel* m_client_model;
     interfaces::Node& m_node;
 
     bool fHaveWatchOnly;
@@ -179,6 +180,7 @@ private:
     interfaces::WalletBalances m_cached_balances;
     EncryptionStatus cachedEncryptionStatus;
     int cachedNumBlocks;
+    QTimer* timer;
 
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();

src/qt/walletview.cpp

@@ -97,6 +97,7 @@ void WalletView::setClientModel(ClientModel *_clientModel)
 
     overviewPage->setClientModel(_clientModel);
     sendCoinsPage->setClientModel(_clientModel);
+    if (walletModel) walletModel->setClientModel(_clientModel);
 }
 
 void WalletView::setWalletModel(WalletModel *_walletModel)