java.lang.NullPointerException:尝试调用虚拟方法“java.lang.String android.os.Bundle.getString（java.lang.String）”

我是Android应用程序开发的初学者。我试图使用intent将评级从第二个活动发送到主活动。执行此操作时，我收到错误：java.lang.NullPointerException:尝试在null上调用虚拟方法“java.lang.String android.os.Bundle.getStringjava.lang.String”

更改以下内容：

enter code here
public class MainActivity extends AppCompatActivity {
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setTextInTextView();
    setContentView(R.layout.activity_main);

}        
public void setTextInTextView(){
    tv = (TextView) findViewById(R.id.rate);
    Bundle extra = getIntent().getExtras();
    tv.setText(""+extra.getString("key"));   
}

public void go(View view) {
    Intent i = new Intent(MainActivity.this,Activity2.class);
    startActivity(i);
}
enter code here
public class Activity2 extends AppCompatActivity {
RatingBar rb;
float str;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_2);
    rb = (RatingBar)findViewById(R.id.rt);


}

public void submit(View view) {
   String st = String.valueOf(rb.getRating());
   Intent i = new Intent(Activity2.this,MainActivity.class);
   i.putExtra("key",""+st);
   startActivity(i);
}
}

为此：

public void setTextInTextView(){
    tv = (TextView) findViewById(R.id.rate);
    Bundle extra = getIntent().getExtras();
    tv.setText(""+extra.getString("key"));   
}

---

感谢Omid的回复，但仍然显示出相同的错误；在setContentViewR.layout.activity_main之后；在那之前，我不知道。。。我得到了它。

public void setTextInTextView(){
    tv = (TextView) findViewById(R.id.rate);
    Bundle extra = getIntent().getExtras();
    if(extra != null)
        tv.setText(""+extra.getString("key"));   
}