kotlin开发安卓app，使用webivew 触发 onShowFileChooser， 但只能触发一次，第二次无法触发，是怎么回事。 如何解决

class MainActivity : AppCompatActivity(), ValueCallback<Array<Uri>> {

    private var mUploadMessage: ValueCallback<Array<Uri>>? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val webView = findViewById<WebView>(R.id.webView)
        webView.webViewClient = WebViewClient()
        webView.webChromeClient = object : WebChromeClient() {
            override fun onShowFileChooser(
                window: Window?,
                filePathCallback: ValueCallback<Array<Uri>>?,
                fileChooserParams: FileChooserParams?
            ): Boolean {
                if (mUploadMessage != null) {
                    mUploadMessage = null
                }

                mUploadMessage = filePathCallback

                // 启动文件选择器
                val intent = fileChooserParams?.createIntent()
                startActivityForResult(intent, FILECHOOSER_RESULT_CODE)

                return true
            }
        }

        // 加载网页
        webView.loadUrl("https://example.com")
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if (requestCode == FILECHOOSER_RESULT_CODE) {
            if (mUploadMessage == null) {
                return
            }

            when (resultCode) {
                Activity.RESULT_OK -> {
                    // 从data Intent中获取Uri
                    val result = if (data == null || data.data == null) {
                        arrayOfNulls<Uri>(0)
                    } else {
                        data.clipData?.let { clipData ->
                            Array(clipData.itemCount) { i ->
                                clipData.getItemAt(i).uri
                            }
                        } ?: arrayOf(data.data!!)
                    }

                    mUploadMessage!!.onReceiveValue(result)
                    mUploadMessage = null
                }
                Activity.RESULT_CANCELED -> {
                    mUploadMessage!!.onReceiveValue(null)
                    mUploadMessage = null
                }
            }
        }
    }

    override fun onReceiveValue(result: Array<Uri>?) {
        // 这里不应该有代码，因为我们只需要传递结果给WebView，而不是处理结果
    }

    companion object {
        private const val FILECHOOSER_RESULT_CODE = 1
    }
}