Null pointer exception when calling a Page Object Model class

I am trying my hands on Selenium test automation using TestNG as the test framework. For this, I have used a Page Object pattern to model each of the page of the website that I am writing the test for.

I have two java classes

1. SignInPage.java (Page Object Model Class)
2. TestLogin.java (where the actual test is written and where the SignInPage is instantiated)

SignInPage.java

public class SignInPage {
private WebDriver driver;

@FindBy(id = "username")
private WebElement usernameTextbox;

@FindBy(id = "password")
private  WebElement passwordTextbox;

@FindBy(xpath = "//*[@id=\"left\"]/div[3]/div/form/input[3]")
private WebElement loginButton;

public SignInPage(WebDriver driver) {
    this.driver = driver;
    driver.get("www.somewebsite.com");
}

public HomePage performLogin(String username, String password){
    usernameTextbox.sendKeys(username);
    passwordTextbox.sendKeys(password);
    loginButton.click();
    return PageFactory.initElements(driver, HomePage.class);
}
}

TestLogin.java

public class TestLogin {
public WebDriver driver;

@BeforeClass
public void setup(){
    System.setProperty("webdriver.chrome.driver", "path_to_chromedriver.exe");
    driver = new ChromeDriver();
}

@Test
public void TestSignIn(){
    SignInPage signInPage = new SignInPage(driver);
    HomePage homePage = signInPage.performLogin("someusername","somepassword");
}
}

My problem is whenever the signInPage.performLogin() is invoked in the @Test of TestLogin.java, a null pointer exception is encountered.

java.lang.NullPointerException
at com.tipidpc.webpages.SignInPage.performLogin(SignInPage.java:30)
at com.tipidpc.tests.TestLogin.TestSignIn(TestLogin.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:104)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:645)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:851)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1177)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:129)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:112)
at org.testng.TestRunner.privateRun(TestRunner.java:756)
at org.testng.TestRunner.run(TestRunner.java:610)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:387)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:382)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:340)
at org.testng.SuiteRunner.run(SuiteRunner.java:289)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1293)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1218)
at org.testng.TestNG.runSuites(TestNG.java:1133)
at org.testng.TestNG.run(TestNG.java:1104)
at org.testng.IDEARemoteTestNG.run(IDEARemoteTestNG.java:72)
at org.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:127)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)