How to avoid NullPointerException

If you make methods and fields static the same result. How to fix this exception?I commented out all sections with exceptions

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Stackoverflow requires more text don't pay attention

DriverManager class

    public class DriverManager {
    private static WebDriver driver;
    public static void openBrowser() {
        System.setProperty("webdriver.chrome.driver", "C:/Users/User/Downloads/chromedriver.exe");
        driver = new ChromeDriver();
        driver.manage().window().maximize();
        driver.navigate().to("http://todomvc.com/examples/react/#/");
        try{
            Thread.sleep(1000);
        }
        catch(InterruptedException ie){
        }
    }

    public static WebDriver getDriver() {
        return driver;
    }

    public static void closeBrowser() {
        driver.close();
    }

    public static void cleanCookies() {
        driver.manage().deleteAllCookies(); //Here exception
    }
    }

BasePage class. All tests here

    public abstract class BasePage {
    protected WebDriver driver;

    public BasePage(){
        PageFactory.initElements(DriverManager.getDriver(), this);
        this.driver = DriverManager.getDriver();
    }
    }

All tests here

    public class TestNGPriority extends BaseTest {
    @Test(priority = 1)
    public void executeTest() {
        MainPage.getInstance().inputValues(); //Here exception
        Assert.assertEquals("firstItem", MainPage.getInstance().verifyStr);
    }
    @Test(priority = 2)
    public void executeTestTwo() {
        MainPage.getInstance().checkAllButton();
        Assert.assertEquals("completed", MainPage.getInstance().verifyCrossOut);
    }

    @Test(priority = 3)
    public void executeTestThree() {
        MainPage.getInstance().clickActiveButton();
        Assert.assertEquals("firstItem", MainPage.getInstance().verifyStr);
    }
    }

BaseTest

    public abstract class BaseTest {
    @BeforeClass
    public static void beforeTestRun () {
        DriverManager.openBrowser();
    }

    @AfterClass
    public static void afterTestSuite() {
        DriverManager.closeBrowser();
    }

    @AfterMethod
    public void cleanCookies() {
        DriverManager.cleanCookies(); //Here exception
    }
    }

MainPage class. All locators here

    public class MainPage extends BasePage {
    @FindBy(xpath = "//input[contains(@class,'new-todo')]")
    private WebElement inputForm;

    @FindBy(xpath = "//ul[contains(@data-reactid,'.0.1.1')]")
    private WebElement findItem;\

    @FindBy(xpath = "//input[contains(@class,'toggle-all')]")
    private WebElement checkAllItems;

    @FindBy(className = "//li[contains(@class,'completed')]")
    private WebElement crossOutItem;

    @FindBy(xpath = "//a[contains(@href,'#/active')]")
    private WebElement activeButton;

    public static MainPage getInstance() {
        return new MainPage(); //Here exception
    }

    public String verifyStr = findItem.getText(); //Here exception
    public String verifyCrossOut = crossOutItem.getText();

    public void inputValues() {
        inputForm.sendKeys("firstItem");
        inputForm.sendKeys(Keys.ENTER);
    }

    public void checkAllButton() {
        checkAllItems.click();
    }

    public void clickActiveButton() {
        activeButton.click();
    }
    }