How to fix the error in enter_text(loc, td) method. dr.findElement(By.xpath(xp)).sendKeys(data) is not working [duplicate]

This code is a program to real data from string and uses it to test in on the website login page. But while I call the enter_txt(loc, td) function in the class driver_script, I get a NullPointer Exception Error.

I think the problem is in the line -

'dr.findElement(By.xpath(xp)).sendKeys(data);'

in 'All_driver_methods' class. But I couldn't figure out it.

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;

public class All_driver_methods {

       WebDriver dr;


    All_driver_methods(WebDriver dr) {

        this.dr = dr;
    }

    public void enter_txt(String xp,String data)
    {
        dr.findElement(By.xpath(xp)).sendKeys(data);
    }

    public void click(String xp)
    {
        dr.findElement(By.xpath(xp)).click();
    }

    public void launchchrome(String url)
    {
        System.setProperty("webdriver.chrome.driver","chromedriver_v78.exe");
        WebDriver dr=new ChromeDriver();              //launch the browser
        dr.get(url);
    }

}

Main Class

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        //login_key ob1=new login_key();
        String kw,loc,td;

        WebDriver dr=null;
        All_driver_methods we=new  All_driver_methods(dr);
        excel_oper excel=new excel_oper();
        for(int r=1;r<=4;r++)
        {

            kw=excel.read_excel(r,3);
            loc=excel.read_excel(r,4);
            td=excel.read_excel(r,5);

            System.out.println(loc);
            System.out.println(kw);
            System.out.println(td);

            switch(kw)
            {
            case "launchchrome":

                we.launchchrome(td);
                break;

            case "Enter_txt":
                we.enter_txt(loc, td); //Error here
                break;

            case "click_btn":
                we.click(loc);
                break;
            }   
        }
    }
}