NullPointerException on setText() method

I have a strange problem I don't understand...

I want to do a PopupWindow from an XML file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/popup" 
android:layout_width="match_parent" 
android:layout_height="match_parent"
android:background="#404040"
android:padding="15px">

<TextView android:id="@+id/popup_title"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    andr开发者_StackOverflow中文版oid:textColor="#ffffff" 
    android:textSize="16dip" 
    android:gravity="center_horizontal" />

</LinearLayout>

and I just want to do a so simple thing: set the text in my TextView... Here is my code:

public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);     

//UI
ui = new RelativeLayout(this);
ui.setBackgroundColor(Color.LTGRAY);
setContentView(ui);

//PopUp
LayoutInflater inflater = (LayoutInflater)this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
popUp = new PopupWindow(this);
popUp.setContentView(inflater.inflate(R.layout.popup_piece, ui, false));


    //Boutton
    bouton = new Button(this);
    bouton.setText("POWPEUP");
    ui.addView(bouton);

    bouton.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {

            popUp.showAtLocation(ui, Gravity.CENTER, 0, 0);
            popUp.update(0, 0, 350, 400);

            titrePopUp = (TextView)findViewById(R.id.popup_title); // THIS RETURNS NULL
            Log.d("TextView", ""+titrePopUp);
                            //titrePopUp.setText("blop", TextView.BufferType.NORMAL); SO THIS DONT WORK
        }
    });
}

Seriously, I don't understand why it returns a NULL value.. Can anyone help me, please?

You are searching for the TextView in your main layout, not in the PopupWindow. You need to do this instead:

titrePopUp = (TextView) popUp.getContentView().findViewById(R.id.popup_titre);

In layout id is spelled as popup_title, and in code it's spelled as popup_titre. Maybe, this is the problem.