关于一个计算器的demo，点击del按键灰闪退

package com.example.demo;

import android.R.string;
 import android.app.Activity;
 import android.os.Bundle;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.View.OnClickListener;
 import android.view.Window;
 import android.widget.Button;
 import android.widget.EditText;
 import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener{
 EditText et;;
 Button btn1;
 Button btn2;
 Button btn3;
 Button btn4;
 Button btn5;
 Button btn6;
 Button btn7;
 Button btn8;
 Button btn9;
 Button btn0;
 Button btn_plus;
 Button btn_minus;
 Button btn_multiply;
 Button btn_devide;
 Button btn_equal;
 Button btn_point;
 Button btn_clear;
 Button btn_del;
 @Override
protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity_main);
    et=(EditText) findViewById(R.id.editText1);
    btn1=(Button) findViewById(R.id.button1);
    btn2=(Button) findViewById(R.id.button2);
    btn3=(Button) findViewById(R.id.button3);
    btn4=(Button) findViewById(R.id.button4);
    btn5=(Button) findViewById(R.id.button5);
    btn6=(Button) findViewById(R.id.button6);
    btn7=(Button) findViewById(R.id.button7);
    btn8=(Button) findViewById(R.id.button8);
    btn9=(Button) findViewById(R.id.button9);
    btn0=(Button) findViewById(R.id.button0);
    btn_point=(Button) findViewById(R.id.button_point);
    btn_clear=(Button) findViewById(R.id.button_c);
    btn_del=(Button) findViewById(R.id.button_del);
    btn_devide=(Button) findViewById(R.id.button_devide);
    btn_equal=(Button) findViewById(R.id.button_equal);
    btn_minus=(Button) findViewById(R.id.button_minus);
    btn_multiply=(Button) findViewById(R.id.button_multiply);
    btn_plus=(Button) findViewById(R.id.button_plus);
    boolean clear_flag=false;
    btn0.setOnClickListener(this);
    btn1.setOnClickListener(this);
    btn2.setOnClickListener(this);
    btn3.setOnClickListener(this);
    btn4.setOnClickListener(this);
    btn5.setOnClickListener(this);
    btn6.setOnClickListener(this);
    btn7.setOnClickListener(this);
    btn8.setOnClickListener(this);
    btn9.setOnClickListener(this);
    btn_plus.setOnClickListener(this);
    btn_minus.setOnClickListener(this);
    btn_multiply.setOnClickListener(this);
    btn_devide.setOnClickListener(this);
    btn_point.setOnClickListener(this);
    btn_equal.setOnClickListener(this);
    btn_clear.setOnClickListener(this);
    btn_del.setOnClickListener(this);
}
@Override
public void onClick(View v) {
    String str=et.getText().toString();
    switch(v.getId()) {
    case R.id.button0:
    case R.id.button1:
    case R.id.button2:
    case R.id.button3:
    case R.id.button4:
    case R.id.button5:
    case R.id.button6:
    case R.id.button7:
    case R.id.button8:
    case R.id.button9:
    case R.id.button_point:
        et.setText(str+((Button)v).getText());
        break;
    case R.id.button_plus:
    case R.id.button_minus:
    case R.id.button_multiply:
    case R.id.button_devide:
        et.setText(str+" "+((Button)v).getText()+" ");
        break;
    case R.id.button_del:
        if(et.getText().toString()!=null&&et.getText().toString()!=""){
            et.setText(str.substring(0, str.length()-1));
        }
        break;
    case R.id.button_c:
        et.setText("");
        break;
    case R.id.button_equal:
        getResult();
        break;
    }
}
private void getResult(){
    double result=0;
    String edt=et.getText().toString();
    if(edt==null||edt==""){
        return;
    }
    if(!edt.contains(" ")){
        return;
    }
    String s1=edt.substring(0,edt.indexOf(" "));
    String op=edt.substring(edt.indexOf(" ")+1,edt.indexOf(" ")+2);
    String s2=edt.substring(edt.indexOf(" ")+3);
    if(s1!=""&&s2!=""){
        double d1=Double.parseDouble(s1);
        double d2=Double.parseDouble(s2);
        if(op.equals("＋")){
            result=d1+d2;
        }
        else if(op.equals("－")){
            result=d1-d2;
        }
        else if(op.equals("×")){
            result=d1*d2;
        }
        else if(op.equals("÷")){
            if(d2==0){
                result=0;
            }else {
                result=d1/d2;
            }
        }
        et.setText(result+"");
    }
    else if(s1!=""&&s2==""){
        et.setText(edt);
    }
    else if(s1==""&&s2!=""){
        double d2=Double.parseDouble(s2);
        if(op.equals("＋")){
            result=d2;
        }
        else if(op.equals("－")){
            result=0-d2;
        }
        else if(op.equals("×")){
            result=0;
        }
        else if(op.equals("÷")){
            result=0;
        }
        et.setText(result+"");
    }
}


}