java.lang.NullPointer Exception (TestNG)

I have a class "BaseTest" class where web driver-related code resides and all the BeforeTest & AfterTest methods. I've extended this method in the 'Login' file and 'SkillsAcquisition' file, but only the login file runs fine and I get NullPointer exception in the second file. I don't know what I'm doing wrong here.

I am new to Selenium and testng so any help is hugely appreciated.

BaseTest.java

package Students;
import org.openqa.selenium.Alert;
import org.openqa.selenium.By;
import org.openqa.selenium.NoAlertPresentException;
import org.openqa.selenium.NoSuchElementException;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.testng.annotations.AfterTest;
import org.testng.annotations.BeforeTest;
public class BaseTest {
String driverPath = "my_path_to_chromedriver";
public String baseUrl = "some_url";
public WebDriver driver ; 
public String expected = null;
public String actual = null;
private boolean acceptNextAlert;
@BeforeTest
public void launchBrowser() {
System.out.println("launching Chrome browser"); 
System.setProperty("webdriver.chrome.driver", driverPath);
driver= new ChromeDriver();
driver.manage().window().maximize();
driver.get(baseUrl);
}
@AfterTest
public void terminateBrowser(){
driver.close();
}
private boolean isElementPresent(By by) {
try {
driver.findElement(by);
return true;
} catch (NoSuchElementException e) {
return false;
}
}
private boolean isAlertPresent() {
try {
driver.switchTo().alert();
return true;
} catch (NoAlertPresentException e) {
return false;
}
}
private String closeAlertAndGetItsText() {
try {
Alert alert = driver.switchTo().alert();
String alertText = alert.getText();
if (acceptNextAlert) {
alert.accept();
} else {
alert.dismiss();
}
return alertText;
} finally {
acceptNextAlert = true;
}
}
}

Login.java

package Students;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import org.openqa.selenium.By;
import org.openqa.selenium.JavascriptExecutor;
import org.openqa.selenium.Keys;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.chrome.ChromeDriver;
import org.testng.Assert;
import org.testng.annotations.BeforeTest;
import org.testng.annotations.Test;
public class Login extends BaseTest{
@Test(priority = 0)
public void login() throws InterruptedException{
driver.findElement(By.id("Username")).clear();
driver.findElement(By.id("Username")).sendKeys("my_username");
driver.findElement(By.id("Password")).clear();
driver.findElement(By.id("Password")).sendKeys("my_password");
driver.findElement(By.id("Password")).sendKeys(Keys.RETURN);
expected = "Expected Text";
actual = driver.getTitle();
Assert.assertEquals(actual, expected);	
}
}

SkillsAcquisition.java

package Students;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import org.openqa.selenium.Alert;
import org.openqa.selenium.By;
import org.openqa.selenium.Keys;
import org.openqa.selenium.NoAlertPresentException;
import org.openqa.selenium.NoSuchElementException;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.testng.Assert;
import org.testng.annotations.AfterMethod;
import org.testng.annotations.AfterTest;
import org.testng.annotations.BeforeMethod;
import org.testng.annotations.BeforeTest;
import org.testng.annotations.Test;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.support.ui.ExpectedConditions;
import org.openqa.selenium.support.ui.Select;
import org.openqa.selenium.support.ui.WebDriverWait;
@Test
public class SkillsAcquisition extends BaseTest{
@Test(priority = 1)
public void addSkills() throws InterruptedException {
System.out.println("Test reached Second file");
WebDriverWait d = new WebDriverWait(driver, 20);			
d.until(ExpectedConditions.visibilityOfAllElementsLocatedBy(By.id("skillAquistionTab")));
driver.findElement(By.id("skillAquistionTab")).click();
Thread.sleep(3000);
d.until(ExpectedConditions.visibilityOfAllElementsLocatedBy(By.xpath("//a[@class='add']")));
Thread.sleep(3000);
driver.findElement(By.xpath("//a[@class='add']")).click();		
System.out.println("Added");
driver.findElement(By.xpath("(//input[@type='text'])[2]")).click();
driver.findElement(By.xpath("(//input[@type='text'])[2]")).clear();
driver.findElement(By.xpath("(//input[@type='text'])[2]")).sendKeys("Skill1");
driver.findElement(By.id("page-wrapper-1")).click();
Thread.sleep(2000);
driver.findElement(By.linkText("Skill1")).click();
Thread.sleep(3000);
driver.findElement(By.id("addProgramButton")).click();
Thread.sleep(2000);
driver.findElement(By.id("ProgramName")).clear();
driver.findElement(By.id("ProgramName")).sendKeys("Program1");
driver.findElement(By.id("SaveNewProgram")).click();
Thread.sleep(2000);
String Newprogramname = driver.findElement(By.linkText("Program1")).getText();
String createdprogramName = "Program1";
Assert.assertEquals(Newprogramname,createdprogramName);
System.out.println("Program created sucessfully");
}	
}

Testng.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Suite">
<test thread-count="5" name="Test">
<classes>
<class name="Students.Login"/>
<class name="Students.SkillsAcquisition"/>
</classes>
</test> 
</suite>

Screenshot of the error.
click to view the image

You need to change @BeforeTest to @BeforeMethod. Before test is executed once for a test tag in your testng.xml. This is why your first test passes but the second fails.

Another option (just FYI but I am sure this is not what you need) is to change your testng.xml to:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE suite SYSTEM "https://testng.org/testng-1.0.dtd">
<suite name="Suite">
  <test thread-count="5" name="Test1">
    <classes>
      <class name="Students.Login"/>
    </classes>
  </test> 
  <test thread-count="5" name="Test2">
    <classes>
      <class name="Students.SkillsAcquisition"/>
    </classes>
  </test> 
</suite>

Since here you have own test tag for each of your test classes, your tests will pass.
Since here you have own test tag for each of your test cla