Use NSOpenPanel from a Mac Catalyst app with the help of Dynamic (https://github.com/mhdhejazi/Dynamic)

Catalyst-NSOpenPanel.swift

// macOS App
let panel = NSOpenPanel()
panel.beginSheetModal(for: view.window!, completionHandler: { response in
    if let url: URL = panel.urls.first {
        print("url: ", url)
    }
})

// Mac Catalyst (with Dynamic)
let panel = Dynamic.NSOpenPanel()
panel.beginSheetModalForWindow(self.view.window!.nsWindow, completionHandler: { response in
    if let url: URL = panel.URLs.firstObject {
        print("url: ", url)
    }
} as ResponseBlock)

typealias ResponseBlock = @convention(block) (_ response: Int) -> Void

UIWindow.nsWindow is defined as follows (source):

extension UIWindow {
    var nsWindow: NSObject? {
        Dynamic.NSApplication.sharedApplication.delegate.hostWindowForUIWindow(self)
    }
}