Android 代码未显示错误，但应用程序已崩溃

应用程序在此自动崩溃，但未显示任何错误

public class MainActivity extends AppCompatActivity {
    Button b1, b2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b1.findViewById(R.id.btn1);
        b2.findViewById(R.id.btn2);


        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast toast1 = Toast.makeText(getApplicationContext(), "I am short", Toast.LENGTH_SHORT);
                toast1.show();
                Intent intent1 = new Intent(Intent.ACTION_VIEW);
                intent1.setData(Uri.parse("http://www.facebook.com"));
                startActivity(intent1);
            }
        });

        b2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Open();

                Toast toast2 = Toast.makeText(getApplicationContext(), "I am here why to fear", Toast.LENGTH_LONG);
                toast2.show();
            }
        });
    }

    public void Open() {
        Intent intent2 = new Intent(this, Main2.class);
        startActivity(intent2);


    }
}

斜体 我找不到此代码中的错误，但应用程序文件未打开。它表示应用程序已停止更改此代码

    b1.findViewById(R.id.btn1);
    b2.findViewById(R.id.btn2)

与

更改此代码

b1.findViewById(R.id.btn1);
b2.findViewById(R.id.btn2);

与

b1.findViewById(R.id.btn1);
b2.findViewById(R.id.btn2);

b1 = (Button)findViewById(R.id.btn1);
b2 = (Button)findViewById(R.id.btn2);