SeleniumProject上的java.lang.NullPointerException

Question

我试图从基类中调用此类，并且发生以下空指针异常。

java.lang.NullPointerException
    at 

SeleniumProject.ReadExcelInput.inputlogin(ReadExcelInput.java:39)

代码：

public class ReadExcelInput extends SearchItem {

    static WebDriver driver;  
    static String email= null;
    static String password = null;


    String ProjectPath = System.getProperty("user.dir"); 
    String filepath = ProjectPath + "/src/testdata";
    String filename = "Login.xls";
    String sheetname ="logincredentials";

    @Test
    public void inputlogin() throws InterruptedException{



    line 39--- Exception--driver.findElement(By.xpath("//*[@id=\'header\']/div[2]/div/div/nav/div[1]/a")).click();

            Thread.sleep(2000);
            try {
                FileInputStream inputStream = new FileInputStream(filepath +"/" + filename);

                // Create workbook instance referencing the file created above
                HSSFWorkbook workbook = new HSSFWorkbook (inputStream);

                // Get your first or desired sheet from the workbook
                Sheet sheet = workbook.getSheetAt(0);