webView的findViewById为null

Question

我正在尝试在Android Studio中创建一个简单的WebView应用程序，但我的WebView为null。我是Android Dev的新手。

MainActivity.java

public class MainActivity extends AppCompatActivity {
 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

            WebView myWebView = (WebView) findViewById(R.id.webview);
// myWebView is null

}

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.patrick.app2.MainActivity">

    <WebView  xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/webview"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        />


</android.support.constraint.ConstraintLayout>