Unable to start activity get a null pointer exception with my code

Question

Hello I'm trying to run a simple Login code in Android. But I get a null pointer exception with my code.

04-21 13:05:58.585 19275-19275/com.example.dell.code E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dell.code/com.example.dell.code.LoginActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
    at android.app.ActivityThread.access$600(ActivityThread.java:141)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5041)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.example.dell.code.LoginActivity.onCreate(LoginActivity.java:42)
    at android.app.Activity.performCreate(Activity.java:5104)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
    at android.app.ActivityThread.access$600(ActivityThread.java:141)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5041)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
    at dalvik.system.NativeStart.main(Native Method)

Can you help me please content_main.xml

<Button
        android:id="@+id/btn_valider"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:layout_marginBottom="20dp"
        android:background="@drawable/colorbutton"
        android:padding="12dp"
        android:text="@string/Connexion"/>

MainActivity.xml

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    ......................................................
    private Button btn_valider;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        btn_valider = (Button) findViewById(R.id.btn_valider);
        btn_valider.setOnClickListener(this);
    }
    @Override
    public void onClick(View v){
        if(v==btn_valider){
            startActivity(new Intent(this, LoginActivity.class));
        }
    }
}

The logcat says that the problem is with the setOnClickListener. I'm not sure what's wrong with it though.

LoginActivity.java

....................................

    public class LoginActivity extends AppCompatActivity implements View.OnClickListener {
        public static final String URL_LOG = "http://10.0.2.2/PHP/login.php";
        public static final String KEY_login = "login";
        public static final String KEY_password = "password";
        private EditText et_login, et_password;
        private Button btn_valider;
        private  String login;
        private  String password;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_login);
            et_login = (EditText) findViewById(R.id.et_login);
            et_password = (EditText) findViewById(R.id.et_password);
            btn_valider = (Button) findViewById(R.id.btn_valider);
            btn_valider.setOnClickListener(this);
        }
        private void userLogin(){
            login = et_login.getText().toString().trim();
            password = et_password.getText().toString().trim();
            StringRequest stringRequest= new StringRequest(Request.Method.POST,URL_LOG,
                    new  Response.Listener<String> () {
                        @Override
                        public void onResponse(String response) {
                            if (response.trim().equals("successs")) {
                                OpenProfile();
                            } else {
                                Toast.makeText(LoginActivity.this, response, Toast.LENGTH_LONG).show();
                            }
                        }
                    },
                    new Response.ErrorListener(){
                        @Override
                    public void onErrorResponse(VolleyError error){
                            Toast.makeText(LoginActivity.this, error.toString(), Toast.LENGTH_LONG).show();
                        }
                    }){
                @Override
            protected Map<String,String> getParams() throws AuthFailureError{
                    Map<String,String> map=new HashMap<String,String>();
                    map.put(KEY_login, login);
                    map.put(KEY_password,password);
                    return map;
                }
            };
            RequestQueue requestqueue= Volley.newRequestQueue(this);
            requestqueue.add(stringRequest);
            }
        private void OpenProfile(){
            Intent intent = new Intent(LoginActivity.this,AccountNavigationActivity.class);
            intent.putExtra(KEY_login,login);
            startActivity(intent);
        }
        public void onClick(View v){userLogin();}

Thanks in advance