即使有来电也要声明为空

Question

我正在尝试检测来电

我已阅读How to detect incoming calls, in an Android device?，但没有帮助。我还读过this tutorial，在我读完所有书后似乎还不错，但是我的String state说意图是null并且没有满足任何if条件，即使手机正在接听电话并且该应用已打开。

我完全按照教程中的说明添加了所有权限，有人可以告诉我我的代码有什么问题吗？

用于检测来电的类：

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.telephony.TelephonyManager;
import android.util.Log;
import android.widget.Toast;

public class PhoneStateReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        Log.d("Something", "something");

        try {
            System.out.println("Receiver start");
            Toast.makeText(context," Receiver start ",Toast.LENGTH_SHORT).show();
            String state = intent.getStringExtra(TelephonyManager.EXTRA_STATE);

            if(state.equals(TelephonyManager.EXTRA_STATE_RINGING)){
                Toast.makeText(context,"Incoming call state", Toast.LENGTH_LONG).show();
                Log("something","something");
            }
            if ((state.equals(TelephonyManager.EXTRA_STATE_OFFHOOK))){
                Toast.makeText(context,"Received State",Toast.LENGTH_SHORT).show();
            }
            if (state.equals(TelephonyManager.EXTRA_STATE_IDLE)){
                Toast.makeText(context,"Idle State",Toast.LENGTH_SHORT).show();
            }
        }
        catch (Exception e){
            e.printStackTrace();
        }
    }
}

清单文件：

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

    <uses-permission android:name="android.permission.INTERNET"></uses-permission>
    <uses-permission android:name="android.permission.ANSWER_PHONE_CALLS"></uses-permission>
    <uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <uses-permission android:name="android.permission.PROCESS_OUTGOING_CALLS"/>

    <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>

        <receiver android:name=".PhoneStateReceiver">
            <intent-filter>
                <action android:name="android.intent.action.PHONE_STATE" />
            </intent-filter>
        </receiver>

    </application>

</manifest>