java之单击按钮后android应用崩溃的原因

我正在尝试制作一个创建对象并将其保存在TreeSet中的应用程序，但是当我按下按钮时，该应用程序将崩溃。我需要帮助

MainActivity如下:

public class MainActivity extends Activity { 
private TreeSet<Variable> arbreDeVariables = new TreeSet<Variable>(); 
 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    creeUneVariable(); 
} 
 
private void creeUneVariable() { 
    Button boutonEnvoyer = (Button)findViewById(R.id.button1); 
    boutonEnvoyer.setOnClickListener(new OnClickListener(){ 
        public void onClick(View v) { 
            Variable variable = new Variable(getUsername(), getPassword()); 
            arbreDeVariables.add(variable); 
        } 
 
    }); 
} 
private String getUsername(){ 
    final EditText username = (EditText)findViewById(R.id.editText1); 
    return username.getText().toString(); 
} 
private String getPassword(){ 
    final EditText password = (EditText)findViewById(R.id.editText2); 
    return password.getText().toString(); 
 
} 
 
 
} 

public class Variable { 
private String username; 
private String password; 
 
public Variable(String username,String password){ 
    this.username = username; 
    this.password = password; 
} 
 
public String getUsername() { 
    return username; 
} 
 
public void setUsername(String username) { 
    this.username = username; 
} 
 
public String getPassword() { 
    return password; 
} 
 
public void setPassword(String password) { 
    this.password = password; 
} 
 
public String toString(){ 
    return "Username: "+username+" Password: "+password; 
} 

请您参考如下方法:

我会说您的editText或button中的一个未命名，就像您在代码中使用它一样。.因此，您将获得NullPointerException