UIDocument not saving

Question

I'm trying to implement a UIDocumentBrowserViewController (new in iOS 11). I can create documents but I can't save them.

My UIDocument subclass overrrides the contents(forType typeName: String) method

override func contents(forType typeName: String) throws -> Any {
    guard let data = content.data(using: .ascii) else { return Data() }
    return data
}

My file editing ViewController gets instantiated with the document and a button press should save the document.

@IBAction func dismissDocumentViewController() {

    document?.content = self.contentTextView.text

    let url = self.document?.fileURL

    self.document?.save(to: url!, for: .forOverwriting, completionHandler: { (success) in
        print("Saved \(success)")
        self.dismiss(animated: true, completion: nil)
    })
}

Calling save does not save the document and I also tried as suggested in this answer: https://stackoverflow.com/a/43571511/4089350

self.document?.close(completionHandler: { (success) in
    print("closed \(success)")
    self.dismiss(animated: true, completion: nil)
})

It prints that it closed the document but it isn't actually saved.

How do I save a UIDocument correctly? Thanks in advance.