On running Applet using applet viewer in Mac, got a few errors

Question

While running an Applet through appletviewer. I used Sublime Text for writing the code and compiled it using "cmd+b". What can be the issue? Is it something related to the applet viewer in particular or related to syntax? I got the following error in my terminal:

java.lang.NullPointerException
    at Calculator.init(Calculator.java:52)
    at sun.applet.AppletPanel.run(AppletPanel.java:435)
    at java.lang.Thread.run(Thread.java:745)

Here's my code:

import java.awt.*; 
import java.awt.event.*; 
import java.applet.*;
import java.lang.*;
import java.io.*;

/*
 * <applet code="Calculator" width=500 height=500></applet>
 * */

public class Calculator extends Applet implements ActionListener
{
    String msg=" "; 
    int v1,v2,result; 
    TextField t1;
    Button b[]=new Button[10];
    Button add,sub,mul,div,clear,mod,EQ; 
    char OP;
    public void init()
    {
        Color k=new Color(10,89,90); 
        setBackground(k);
        t1=new TextField(50);
        GridLayout gl=new GridLayout(6,3); 
        setLayout(gl);
        for(int i=0;i<i++;)
        {
            b[i]=new Button(""+i);
        }
        add=new Button("+"); 
        sub=new Button("-"); 
        mul=new Button("*"); 
        div=new Button("/"); 
        mod=new Button("%"); 
        clear=new Button("Clear"); 
        EQ=new Button("="); 
        t1.addActionListener(this);
        add(t1);
        for(int i=0;i<i++;)
        {
            add(b[i]);
        }
        add(add);
        add(sub);
        add(mul);
        add(div);
        add(mod);
        add(clear);
        add(EQ);
        for(int i=0;i<10;i++)
        {
            b[i].addActionListener(this);
        }
        add.addActionListener(this);
        sub.addActionListener(this);
        mul.addActionListener(this);
        div.addActionListener(this);
        mod.addActionListener(this);
        clear.addActionListener(this);
        EQ.addActionListener(this);
    }
    public void actionPerformed(ActionEvent ae)
    {
        String str=ae.getActionCommand(); 
        char ch=str.charAt(0);
        
        if ( Character.isDigit(ch)) 
            t1.setText(t1.getText()+str); 
        else
            if(str.equals("+"))
            {
                v1=Integer.parseInt(t1.getText());
                OP='+';
                t1.setText("");
            }
            else if(str.equals("-"))
            {
                v1=Integer.parseInt(t1.getText()); OP='-';
                t1.setText("");
            } 
            else if(str.equals("*"))
            {
                v1=Integer.parseInt(t1.getText());
                OP='*';
                t1.setText("");
            }
            else if(str.equals("/"))
            {
                v1=Integer.parseInt(t1.getText());
                OP='/';
                t1.setText("");
            }
            else if(str.equals("%"))
            {
                v1=Integer.parseInt(t1.getText());
                OP='%';
                t1.setText("");
            }
        
        if(str.equals("="))
        {
            v2=Integer.parseInt(t1.getText());
            if(OP=='+')
                result=v1+v2; 
            else if(OP=='-')
                result=v1-v2; 
            else if(OP=='*')
                result=v1*v2; 
            else if(OP=='/')
                result=v1/v2; 
            else if(OP=='%')
                result=v1%v2;
            t1.setText(""+result);
        }
        if(str.equals("Clear"))
        {
            t1.setText("");
        }
    }
}

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