[qt] Don't call method on null WalletModel object

This doesn't crash currently because the method doesn't access any object
members, but this behavior is fragile and incompatible with #10102.

src/qt/sendcoinsdialog.cpp

@@ -115,7 +115,6 @@ SendCoinsDialog::SendCoinsDialog(const PlatformStyle *_platformStyle, QWidget *p
     ui->groupCustomFee->button((int)std::max(0, std::min(1, settings.value("nCustomFeeRadio").toInt())))->setChecked(true);
     ui->customFee->setValue(settings.value("nTransactionFee").toLongLong());
     ui->checkBoxMinimumFee->setChecked(settings.value("fPayOnlyMinFee").toBool());
-    ui->optInRBF->setCheckState(model->getDefaultWalletRbf() ? Qt::Checked : Qt::Unchecked);
     minimizeFeeSection(settings.value("fFeeSectionMinimized").toBool());
 }
 
@@ -176,6 +175,9 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         updateSmartFeeLabel();
         updateGlobalFeeVariables();
 
+        // set default rbf checkbox state
+        ui->optInRBF->setCheckState(model->getDefaultWalletRbf() ? Qt::Checked : Qt::Unchecked);
+
         // set the smartfee-sliders default value (wallets default conf.target or last stored value)
         QSettings settings;
         if (settings.value("nSmartFeeSliderPosition").toInt() == 0)