Android Studio：错误：找不到符号变量activity_main

Question

当我尝试运行this程序时，我得到this错误（找不到符号变量activity_main） This是我的AndroidManifest.xml This是我的activity_main.xml

代码：MainActivity.java：

public class MainActivity extends AppCompatActivity {

    Button button;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);
        bgElement.setBackgroundColor(Color.WHITE);
        ButtonListCheck();
    }


    public void ButtonListCheck() {
        button = (Button) findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                RelativeLayout bgElement;
                bgElement = (RelativeLayout)
                        findViewById(R.id.activity_main);
                int color = ((ColorDrawable)
                        bgElement.getBackground()).getColor();
                if (color == Color.RED) {
                    bgElement.setBackgroundColor(Color.BLUE);
                } else {
                    bgElement.setBackgroundColor(Color.RED);
                }
            }
        });
    }
}

代码activity_main.xml：

<?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=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/change" />
</android.support.constraint.ConstraintLayout>

代码AndroidManifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.razvan.fckingbutton">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

我已经尝试过使缓存无效/重启   请帮帮我！