Android Unfortunately Hatası ...

Android Studıo da WhatsApp Benzeri bi uygulama yapıyorum ve bu hatayı alıyorum android studıo hakkında bilgisi olan varsa çözüm olarak yazarsa sevinirim.(Emulatör çalıştırıken böyle hata cıkıo ama buidgradle programın duzgun calıstıgını yeşil tik olarak gosterıo)Altta kodları verecegım; 

Main Activity kodlarım  

package com.example.chatapp;

import android.content.Intent;
import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.MenuInflater;

import com.google.firebase.auth.FirebaseUser;

public class MainActivity extends AppCompatActivity {

    private Toolbar mToolbar;
    private ViewPager myViewPager;
    private TabLayout myTabLayout;
    private SekmeErisimAdapter mySekmeErisimAdapter;

    //Firebase
    private FirebaseUser mevcutKullanici;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mToolbar=findViewById(R.id.ana_sayfa_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setTitle("ChatApp");

        myViewPager=findViewById(R.id.ana_sekmeler_pager);
        mySekmeErisimAdapter=new SekmeErisimAdapter(getSupportFragmentManager());
        myViewPager.setAdapter(mySekmeErisimAdapter);

        myTabLayout=findViewById(R.id.ana_sekmeler);
        myTabLayout.setupWithViewPager(myViewPager);
    }

    @Override
    protected void onStart() {
        super.onStart();

        if (mevcutKullanici ==null)
        {
            KullaniciyiLoginActivityeGonder();
        }
    }

    private void KullaniciyiLoginActivityeGonder()
    {
        Intent loginIntent = new Intent(MainActivity.this,LoginActivity.class);
        startActivity(loginIntent);
    }
}

Hata Görüntüsü

12-04 06:04:16.152 3740-3740/? E/libprocessgroup: failed to make and chown /acct/uid_10060: Read-only file system
12-04 06:04:16.684 3740-3740/com.example.chatapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.chatapp, PID: 3740
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chatapp/com.example.chatapp.MainActivity}: java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead.
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead.
        at android.support.v7.app.AppCompatDelegateImpl.setSupportActionBar(AppCompatDelegateImpl.java:345)
        at android.support.v7.app.AppCompatActivity.setSupportActionBar(AppCompatActivity.java:130)
        at com.example.chatapp.MainActivity.onCreate(MainActivity.java:30)
        at android.app.Activity.performCreate(Activity.java:5990)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387) 
        at android.app.ActivityThread.access$800(ActivityThread.java:151) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:135) 
        at android.app.ActivityThread.main(ActivityThread.java:5254) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at java.lang.reflect.Method.invoke(Method.java:372) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698) 

Android Manifest Görüntüsü

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

    <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/Theme.AppCompat">
        <activity android:name=".KayitActivity"></activity>
        <activity android:name=".LoginActivity" />
        <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>

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