qt: Fix regression in TransactionTableModel

Summary:
Since PR17993 a crash is possible on exit.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>

This is a backport of [[bitcoin-core/gui#8 | core-gui#8]]

Depends on D9254

Test Plan:
start bitcoin-qt
wait until sync
on main window: Menu -> File -> Quit

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9257

src/qt/transactionrecord.cpp

@@ -214,6 +214,7 @@ void TransactionRecord::updateStatus(const interfaces::WalletTxStatus &wtx,
 }
 
 bool TransactionRecord::statusUpdateNeeded(const BlockHash &block_hash) const {
+    assert(!block_hash.IsNull());
     return status.m_cur_block_hash != block_hash;
 }
 

src/qt/transactiontablemodel.cpp

@@ -186,7 +186,8 @@ class TransactionTablePriv {
             interfaces::WalletTxStatus wtx;
             int numBlocks;
             int64_t block_time;
-            if (rec->statusUpdateNeeded(cur_block_hash) &&
+            if (!cur_block_hash.IsNull() &&
+                rec->statusUpdateNeeded(cur_block_hash) &&
                 wallet.tryGetTxStatus(rec->txid, wtx, numBlocks, block_time)) {
                 rec->updateStatus(wtx, cur_block_hash, numBlocks, block_time);
             }
@@ -671,9 +672,8 @@ QVariant TransactionTableModel::headerData(int section,
 QModelIndex TransactionTableModel::index(int row, int column,
                                          const QModelIndex &parent) const {
     Q_UNUSED(parent);
-    TransactionRecord *data =
-        priv->index(walletModel->wallet(),
-                    walletModel->clientModel().getBestBlockHash(), row);
+    TransactionRecord *data = priv->index(
+        walletModel->wallet(), walletModel->getLastBlockProcessed(), row);
     if (data) {
         return createIndex(row, column, data);
     }

src/qt/walletmodel.cpp

@@ -72,7 +72,7 @@ void WalletModel::pollBalanceChanged() {
     // Avoid recomputing wallet balances unless a TransactionChanged or
     // BlockTip notification was received.
     if (!fForceCheckBalanceChanged &&
-        m_cached_last_update_tip == m_client_model->getBestBlockHash()) {
+        m_cached_last_update_tip == getLastBlockProcessed()) {
         return;
     }
 
@@ -531,3 +531,7 @@ bool WalletModel::isMultiwallet() {
 const CChainParams &WalletModel::getChainParams() const {
     return Params();
 }
+
+BlockHash WalletModel::getLastBlockProcessed() const {
+    return m_client_model ? m_client_model->getBestBlockHash() : BlockHash{};
+}

src/qt/walletmodel.h

@@ -162,6 +162,8 @@ class WalletModel : public QObject {
         return addressTableModel;
     }
 
+    BlockHash getLastBlockProcessed() const;
+
 private:
     std::unique_ptr<interfaces::Wallet> m_wallet;
     std::unique_ptr<interfaces::Handler> m_handler_unload;