亲们，谁能帮我解答下为什么会出现此类异常？ Exception in thread "main" java.lang.NullPointerException at com.imooc.collection.SetTest.testAdd(SetTest.java:19) at com.imooc.collection.SetTest.main(SetTest.java:52) 19：Course cr1=new Course("1","数据结构");  coursesToSelect.add(cr1); 52：SetTest st=new SetTest(); st.testAdd();