Getting NullPointerException while running script with Testng xml but script is working fine when running with "Running Testng Programatically"

i'm facing one different issue that when i'm running my script with Testng xml, it is throwing null point error but when i'm running script with "running Testng Programatically" it is working fine with out any issue.

Code

public void TestCaseExecutor(Class classname) throws ClassNotFoundException {

    FunctionLibrary lib = new FunctionLibrary(driver);

    // Getting TestName
    for (int Tests = 1; Tests < TestData.sheet.getLastRowNum() + 1; Tests++) {
        String ClassName = classname.getSimpleName();

        String TestName = TestData.sheet
                            .getRow(Tests)
                            .getCell(0)
                            .getStringCellValue()
                            .trim();

        // Comparing TestNames from sheet
        if (ClassName.equals(TestName)) {
            // Method Name from Excel
            String MethodName = TestData.sheet
                                    .getRow(Tests)
                                    .getCell(2)
                                    .getStringCellValue()
                                    .trim();
            try {
                // parameter count from excel
                int parameterCount = (int) TestData.sheet
                                            .getRow(Tests)
                                            .getCell(3)
                                            .getNumericCellValue();

                // reading Method names from Functional library
                for (Method m : FunctionLibrary.class.getMethods()) {
                    // Comparing Method Name with Excel method name
                    if (m.getName().equals(MethodName)) {
                        // Comparing paraameter Count from both FL and Excel
                        if (m.getParameterCount() == parameterCount) {
                            // Creating an array with class
                            Class<?> param[] = new Class[parameterCount];

                            for (int i = 0; i < m.getParameterCount(); i++) {
                                // assigning values in to array with parameter type
                                param[i] = m.getParameterTypes()[i];
                            }

                            Method method = FunctionLibrary.class
                                    .getMethod(MethodName, param);
                            method.invoke(lib, FunctionLibrary
                                    .ParameterData(parameterCount, Tests));
                        }
                    } else if (m.getName().equals("")) {
                        System.out.println("empty method name");
                        break;
                    }
                }
            } catch (InvocationTargetException 
                    | NoSuchMethodException 
                    | IllegalAccessException
                    | NullPointerException e) {
                e.printStackTrace();
                assertTrue(false);
            }
        }
    }
}

calling the above function in my script

public class TestCase3_Register extends Browser_nav{

    @Test
    public void register() {
        FunctionLibrary obj = new FunctionLibrary(driver);

        try {
            obj.TestCaseExecutor(this.getClass());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } 
    }
}

My Excel Class

public class LoadExcel {

    public static int rowCount;
    public static XSSFSheet sheet;
    public static XSSFWorkbook book;
    public static void loadExcelSheet(){
        try{

        File file=new File("C:\\Users\\DELL\\Desktop\\TestData.xlsx");
        FileInputStream fis=new FileInputStream(file);
        try {
            book=new XSSFWorkbook(file);
            sheet=book.getSheet("TestSteps");
            rowCount=sheet.getLastRowNum();
        } catch (InvalidFormatException | IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        }catch(FileNotFoundException e){
            e.printStackTrace();
        }
    }

}

Error log:

java.lang.NullPointerException
at lib.FunctionLibrary.TestCaseExecutor(Functionlibrary.java:50)
at testCaseWithKeywordFramework.TestCase3_Register(TestCase_Register.java:21)
at org.testng.remote.AbstractRemoteTestNG.run(AbstractRemoteTestNG.java:114)

is there anybody have an idea to how to solve this thing.