A user reported to me that they are unable to create a PSBT with a legacy watchonly wallet when they use the GUI. They get an error "The amount exceeds your balance".

I think the issue is that the GUI will retrieve the wallet's balance via `AvailableCoins` before calling `CreateTransaction`. This call to `AvailableCoins` is resulting in fewer coins being returned with recent changes to how `AvailableCoins` works, particularly with additional filtering of spendable coins and preset coins.

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Note that this does not effect descriptor wallets. A workaround for this issue is to switch to using descriptor wallets. An existing legacy wallet can be turned into a descriptor wallet using the `migratewallet` RPC. However many watchonly legacy wallets are likely the result of importing descriptors via `importmulti`. For such wallets, I recommend creating a new descriptor wallet and re-importing the descriptors.
{
    this->model = _model;
    if(_model && _model->getOptionsModel())
    {
        for(int i = 0; i < ui->entries->count(); ++i)
        {
            SendCoinsEntry *entry = qobject_cast<SendCoinsEntry*>(ui->entries->itemAt(i)->widget());
            if(entry)
            {
                entry->setModel(_model);
            }
        }
        connect(_model, &WalletModel::balanceChanged, this, &SendCoinsDialog::setBalance);
        connect(_model->getOptionsModel(), &OptionsModel::displayUnitChanged, this, &SendCoinsDialog::refreshBalance);
        refreshBalance();
        
        connect(_model->getOptionsModel(), &OptionsModel::displayUnitChanged, this, &SendCoinsDialog::coinControlUpdateLabels);
        connect(_model->getOptionsModel(), &OptionsModel::coinControlFeaturesChanged, this, &SendCoinsDialog::coinControlFeatureChanged);
        ui->frameCoinControl->setVisible(_model->getOptionsModel()->getCoinControlFeatures());
        coinControlUpdateLabels();
        
        for (const int n : confTargets) {
            ui->confTargetSelector->addItem(tr("%1 (%2 blocks)").arg(GUIUtil::formatNiceTimeOffset(n*Params().GetConsensus().nPowTargetSpacing)).arg(n));
        }
        connect(ui->confTargetSelector, qOverload<int>(&QComboBox::currentIndexChanged), this, &SendCoinsDialog::updateSmartFeeLabel);
        connect(ui->confTargetSelector, qOverload<int>(&QComboBox::currentIndexChanged), this, &SendCoinsDialog::coinControlUpdateLabels);