how to resolve Attempt to de-reference a null object

Question

the code compiles but the page has Attempt to de-reference a null object.I tried system.assertequals so myTeacher and teacherSet is not null but myTea is null. it does not return error and i have no clue what went wrong. (hardcoded selectedSId for testing)

 public void fetchTeacherInfo(){

            selectedSId = 'a0P28000001pOBa';
                for (student__c stud : [Select Id, name, first_name__c, parent__c, teacher_set__c from student__c WHERE Id = : selectedSId]){   
                    teacherSet = stud.teacher_set__c;
                           }
                 for (teacher__c myTeacher : [Select Id, name, first_name__c, teacher_set__c, middle_name__c from teacher__c WHERE teacher_set__c = : teacherSet]){
                 myTea.add(myTeacher.id);

                 if (myTeacher !=  null)
                 {


                            //  system.assertequals(myTea + 'test','test');

                 }
             }
          }     

Answer 1

You need to initialize your set before adding value in it..

In your constructor

myTea = new set<Id>();

after that you can add value in this set