Error using radiobutton getText () method

I created a very simple app to calculate a bill with tip add where I enter a value, select a Radio Button with a percentage and it generates the final value as a result.

But every time I click on "Calculate" it stops working and closes with the error:

  

FATAL EXCEPTION: main Process: com.exercise.tipcalc, PID: 1870 java.lang.IllegalStateException: Could not execute method for android: onClick>
  ......

Follow the code below:

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

private EditText input;
private TextView output;
private RadioGroup rg;
private RadioButton rb;
private Button calcular;
private int op = 0;
private double valor = 0;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    input = (EditText) findViewById(R.id.input);
    output = (TextView) findViewById(R.id.output);
    rg = (RadioGroup) findViewById(R.id.rg);
    calcular = (Button) findViewById(R.id.calcular);
}



   public void calcular(View view){
        op = rg.getCheckedRadioButtonId();
        valor = Double.parseDouble(input.getText().toString());
        //output.setText(input.getText());
        if(rb.getText().equals("10%")){
            output.setText(String.valueOf(valor+(valor*.10)));
        }else if(rb.getText().equals("15%")){
            output.setText(String.valueOf(valor+(valor*.15)));
        }else if(rb.getText().equals("20%")){
            output.setText(String.valueOf(valor+(valor*.20)));
        }
    }

}

This is the XML of my screen:

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/output"
    android:text="carambola"
    android:layout_below="@+id/calcular"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="25dp" />

<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/input"
    android:layout_below="@+id/titulo"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Calcular"
    android:id="@+id/calcular"
    android:layout_below="@+id/rg"
    android:layout_centerHorizontal="true"
    android:onClick="calcular" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="Informe o valor da conta"
    android:id="@+id/titulo"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Escolha a porcentagem da gorjeta"
    android:id="@+id/titulo2"
    android:layout_below="@+id/input"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="25dp" />

<RadioGroup
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/rg"
    android:layout_marginTop="31dp"
    android:layout_below="@+id/titulo2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true">

<RadioButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="10%"
    android:id="@+id/val1"
    android:layout_below="@+id/titulo2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:checked="false" />

<RadioButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="15%"
    android:id="@+id/val2"
    android:layout_below="@+id/val1"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:checked="false" />

<RadioButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="20%"
    android:id="@+id/val3"
    android:layout_below="@+id/val2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:checked="false"
    android:onClick="calcular" />
</RadioGroup>