Driver keeps saying "cannot read the array length because "this.grades" is null

Question

Here is the class I am calling in the driver:

public class CourseGrades{

private final int NUMBER_OF_GRADES = 4; private GradedActivity[] grades;

  public CourseGrades(){
  }

  public void setLab(GradedActivity gradedActivity){
    grades[0] = gradedActivity;
    gradedActivity.setScore(85);
  }

  public void setPassFailExam(PassFailExam passFailExam){
    grades[1] = passFailExam;
    passFailExam.setScore(85);
  }

  public void setEssay(Essay essay){
    grades[2] = essay;
    essay.setScore(15, 15, 15, 30);
  }

  public void setFinalExam(FinalExam finalExam){
    grades[3] = finalExam;
    finalExam.setScore(80);
  }


  @Override
  public String toString() {
    String retVal = "";

    for (int index = 0; index < grades.length; index++) {
      retVal += (grades[index].getGrade() + "\n" +
        grades[index].getGrade() + "\n");

    }

    return retVal;
  }
}

Here is my driver:

public class CourseGradesDemo {
  public static void main(String[] args) {

    CourseGrades courseGrades = new CourseGrades();

    System.out.println(courseGrades);

  }
}

I tried giving the array a length but it kept giving me errors.