为什么要试试！ NSAttributedString（data：options：documentAttributes :)失败了吗？

Question

因此，大约1％的用户会话在所有iOS版本和设备上都出现以下错误：

1    libswiftCore.dylib    specialized _assertionFailure(_:_:file:line:flags:)
2    libswiftCore.dylib    swift_unexpectedError
3    PCC2                  closure #2 in onLoad() //this is the line that's highlighted
4    PCC2                  specialized closure #1 in backgroundThread(_:background:completion:)
5    PCC2                  closure #1 in backgroundThread(_:background:completion:)
6    PCC2                  thunk for @calee_owned () -> ()

导致此错误的代码是：

backgroundThread(background: {
    let _ = try! NSAttributedString( // this is the line that the highlighted line leads to
        data: "<b>bold!</b>".data(using: String.Encoding.unicode, allowLossyConversion: true)!,
        options: [.documentType: NSAttributedString.DocumentType.html, .characterEncoding: String.Encoding.utf8.rawValue],
        documentAttributes: nil)
    }, completion: {
    attributedStringsReady = true
})

backgroundThread如下：

func backgroundThread(_ delay: Double = 0.0, background: (() -> Void)? = nil, completion: (() -> Void)? = nil) {
    DispatchQueue.global(qos: .userInitiated).async {
        if(background != nil){ background!(); }

        let popTime = DispatchTime.now() + Double(Int64(delay * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC)
        // the above line is what Crashlytics points to
        DispatchQueue.main.asyncAfter(deadline: popTime) {
            if(completion != nil){ completion!(); }
        }
    }
}

不知道如何重现这一点。我甚至不知道如何从管理器中复制痕迹，我不得不重新输入整个内容。