Using master (98958c81f5065a5de13699d46995d278ecb6709e) with Qt `5.12.3` (also tested with depends) with multiple wallets available. None are shown in the `Open Wallet ->` menu in the gui. The wallets can be loaded from the console, bitcoin-cli etc.
{
    Q_OBJECT
public:
    static const std::string DEFAULT_UIPLATFORM;
    explicit BitcoinGUI(interfaces::Node& node, const PlatformStyle *platformStyle, const NetworkStyle *networkStyle, QWidget *parent = nullptr);
    ~BitcoinGUI();
    /** Set the client model.
        The client model represents the part of the core that communicates with the P2P network, and is wallet-agnostic.
    */
    void setClientModel(ClientModel *clientModel);
#ifdef ENABLE_WALLET
    void setWalletController(WalletController* wallet_controller);
#endif
#ifdef ENABLE_WALLET
    /** Set the wallet model.
        The wallet model represents a bitcoin wallet, and offers access to the list of transactions, address book and sending
        functionality.
    */
    void addWallet(WalletModel* walletModel);
    void removeWallet(WalletModel* walletModel);
    void removeAllWallets();
#endif 
    bool enableWallet = false;
    /** Get the tray icon status.
        Some systems have not "system tray" or "notification area" available.
    */
    bool hasTrayIcon() const { return trayIcon; }