fileExporter modifier does not compile when passing document from property

I try to use the SwiftUI fileExporter modifier to save different kinds of documents from my app on macOS. Therefore, I store the document to save in an optional FileDocument? property.

import SwiftUI
import UniformTypeIdentifiers

struct PlaintextFile: FileDocument {

    static var readableContentTypes = [UTType.plainText]
    var text: String = ""

    init(text: String) {
        self.text = text
    }

    init(configuration: ReadConfiguration) throws {
        if let data = configuration.file.regularFileContents {
            text = String(decoding: data, as: UTF8.self)
        }
    }

    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper {
        let data = Data(text.utf8)
        return FileWrapper(regularFileWithContents: data)
    }
}


struct ContentView: View {
    @State private var showFileExporter = false
    private let document: FileDocument? = PlaintextFile(text: "")

    var body: some View {
        Button("Save File: ") {
            self.showFileExporter = true
        }
        .padding()
        .fileExporter(isPresented: self.$showFileExporter, document: self.document, contentType: .plainText) { _ in }
    }
}

However, this sample code above does not compile unless I initialize PlaintextFile directly in the fileExporter call. With the sample code the compiler shows the error

ContentView.swift:37:10: No exact matches in call to instance method 'fileExporter' /SwiftUI.View:5:17: Candidate requires that 'FileDocument' conform to 'FileDocument' (requirement specified as 'D' == 'FileDocument')

Is there a way to not specify the document to save directly in the modifier but maintain it as optional property?