checking if a radio button is selected

i have a radio button group which contains 4 radio buttons none of them is checked by default

  <RadioGroup
        android:id="@+id/radioGroup1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp" >

        <RadioButton
            android:id="@+id/ans11"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/q"
            android:text="RadioButton"
            android:textColor="#747577"
            android:textSize="35dp" />

        <RadioButton
            android:id="@+id/ans22"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/ans11"
            android:text="RadioButton"
            android:textColor="#747577"
            android:textSize="35dp" />

        <RadioButton
            android:id="@+id/ans33"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_above="@+id/memberexdone"
            android:layout_below="@+id/ans22"
            android:text="RadioButton"
            android:textColor="#747577"
            android:textSize="35dp" />

        <RadioButton
            android:id="@+id/ans44"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_above="@+id/memberexdone"
            android:layout_below="@+id/ans22"
            android:text="RadioButton"
            android:textColor="#747577"
            android:textSize="35dp" />
    </RadioGroup>

im trying to check if clicked on a button without selecting any of the radio buttons ..

  rg = (RadioGroup) findViewById(R.id.radioGroup1);

            // get selected radio button from radioGroup
            int selectedId = rg.getCheckedRadioButtonId();

            // find the radiobutton by returned id
            rbtn = (RadioButton) findViewById(selectedId);

            String selected = rbtn.getText().toString();
            if (rg.getCheckedRadioButtonId() == -1)
            {
                //if the password less than 8 or more than 10
                AlertDialog.Builder alertBuilder3=new AlertDialog.Builder(MemberExercise.this);
                alertBuilder3.setTitle("Invalid");
                alertBuilder3.setMessage("Please select an answer first, then click on done.");
                alertBuilder3.setPositiveButton("Ok", new DialogInterface.OnClickListener() {

                    public void onClick(DialogInterface dialog, int which) {
                    dialog.cancel();


                    }
                });

                alertBuilder3.create().show();
            }
            else
            {
                .....
            }

also tried if (rbtn.equals("")){}

but both when i click on the button without selecting any .. the application will stop ..why ?