Android Studio切换开关，提供空对象引用

Question

所以我试图在Android Studio中使用Switch切换小部件并且它提供了一个空对象引用，这是我正在使用的代码。

`import android.content.DialogInterface;
import android.content.Intent;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.CompoundButton;
import android.widget.Switch;
import android.widget.TextView;

public class ProfileActivity extends AppCompatActivity implements CompoundButton.OnCheckedChangeListener {


    TextView username;
    Switch sw1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        sw1 = (Switch)findViewById(R.id.switch1);
        final String usersUsername = getIntent().getExtras().getString("username");
        Log.d("USERNAME",usersUsername);
        setContentView(R.layout.activity_profile);
        username=(TextView)findViewById(R.id.userNameTxt);
        username.append(usersUsername);
        sw1.setOnCheckedChangeListener(this);

    }
    @Override
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
        if (sw1.isChecked()){
            AlertDialog.Builder dialog = new AlertDialog.Builder(ProfileActivity.this);
            dialog.setCancelable(false);
            dialog.setTitle("Turn on notifications");
            dialog.setMessage("Are you sure you want to turn notifications on?" );
            dialog.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int id) {
                    //Action for "OK".
                    sw1.setChecked(true);
                }
            });
            dialog.setNegativeButton("cancel", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialogInterface, int i) {
                    sw1.setChecked(false);
                }
            });
        }

        else {
            sw1.setChecked(false);
        }
    }



}

当我尝试运行此操作时出现此错误：Attempt to invoke virtual method 'void android.widget.Switch.setOnCheckedChangeListener(android.widget.CompoundButton$OnCheckedChangeListener)' on a null object reference

我不知道如何为对象设置任何内容以使其无效，我做错了什么？