ارور ONCLICK در ماشین حساب

package com.sarimatix.samira.calculator;

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

public class MainActivity extends AppCompatActivity {
    TextView txtmini;
    TextView txtresult;
  Button btn0;
  Button btn1;
  Button btn2;
  Button btn3;
  Button btn4;
  Button btn5;
  Button btn6;
  Button btn7;
  Button btn8;
  Button btn9;
  Button btnplus;
  Button btnmultiply;
  Button btnmines;
  Button btndivision;
  Button btnpoint;
  Button btnclean;
  Button btncleanentity;
  Button btnback;
  Button btnsign;
  Button btnequal;
  boolean resetlcd = false;
  double currentresult =0;
  String operator="";


 public void backOne(){
    String back=txtresult.getText().toString();
    String newBack=back.substring(0,back.length()-1);
    txtresult.setText(newBack);  }

 public void point(){
    if (resetlcd){
    txtresult.setText("0");
    resetlcd=false;}
    String oldvalue =txtresult.getText().toString();
    if (oldvalue.contains("."))
    {return;}
    if (oldvalue.length()>10)
    {return; }
    txtresult.setText(oldvalue + ".");  }

 public void compiute(String nextOperator){
    String resultvalue =txtresult.getText().toString();
    double resultnumber=Double.parseDouble(resultvalue);
    if(operator.equals("+")){
    currentresult += resultnumber;}
    else if(operator.equals("-")){
    currentresult -= resultnumber;}
    else if(operator.equals("/")){
    currentresult /= resultnumber;}
    else if(operator.equals("*")){
    currentresult *= resultnumber;}
    else if(operator.equals("")){
    currentresult = resultnumber;}
    String oldresult=txtmini.getText().toString();
    txtmini.setText(oldresult+resultnumber+nextOperator);
    txtresult.setText("" + currentresult);
    operator=nextOperator;
    resetlcd = true;  }

 public void  prosessEqual() {
   compiute("");
   txtmini.setText(""); }

 public void appendOperator(String nextOperator){
    compiute(nextOperator);}


 public void appendNumber(int number){
    String oldvalue =txtresult.getText().toString();
    if (resetlcd){
    oldvalue ="";
    resetlcd=false;}
      if(oldvalue.equals("0")){
      if (number==0)
      {return; }
      oldvalue ="";}
      if (oldvalue.length()>10)
      {return;}
    txtresult.setText(oldvalue + number);  }

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

      txtmini=(TextView)findViewById(R.id.txtmini);
      txtresult=(TextView)findViewById(R.id.txtresult);
    btn0=(Button)findViewById(R.id.btn0);
    btn1=(Button)findViewById(R.id.btn1);
    btn2=(Button)findViewById(R.id.btn2);
    btn3=(Button)findViewById(R.id.btn3);
    btn4=(Button)findViewById(R.id.btn4);
    btn5=(Button)findViewById(R.id.btn5);
    btn6=(Button)findViewById(R.id.btn6);
    btn7=(Button)findViewById(R.id.btn7);
    btn8=(Button)findViewById(R.id.btn8);
    btn9=(Button)findViewById(R.id.btn9);
    btnplus=(Button)findViewById(R.id.btnplus);
    btnmines=(Button)findViewById(R.id.btnmines);
    btnmultiply=(Button)findViewById(R.id.btnmultiply);
    btndivision=(Button)findViewById(R.id.btndivision);
    btnpoint=(Button)findViewById(R.id.btnpoint);
    btnclean=(Button)findViewById(R.id.btnclean);
    btncleanentity=(Button)findViewById(R.id.btncleanentity);
    btnback=(Button)findViewById(R.id.btnback);
    btnsign=(Button)findViewById(R.id.btnsign);
    btn0.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(0);
        }
    });
    btn1.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(1);
        }
    });
    btn2.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(2);
        }
    });
    btn3.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(3);
        }
    });
    btn4.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(4);
        }
    });
    btn5.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(5);
        }
    });
    btn6.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(6);
        }
    });
    btn7.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(7);
        }
    });
    btn8.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(8);
        }
    });
    btn9.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendNumber(9);
        }
    });
    btnplus.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendOperator("+");
        }
    });
    btnmines.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendOperator("-");
        }
    });
    btnmultiply.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendOperator("*");
        }
    });
    btndivision.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        appendOperator("/");
        }
    });
    btnpoint.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        point();
        }
    });
    btnclean.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
       txtresult.setText("0");
        txtmini.setText("");
        currentresult=0;
        resetlcd=false;
        }
    });
    btncleanentity.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
       txtresult.setText("0");
       }
    });
    btnback.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        backOne();
        }
    });
    btnequal.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        prosessEqual();
      }
    });

  }
}