You need to show us more about NewClass. Do you understand what abstract methods are? The NewClass has a method which has a name, has the identifier abstract and has no implementation or code. It is a sign that subclasses, like NewGUI, must write the code for that method. You need to find out which method is missing and what it is supposed to do. Then you need to implement (write the code) for that method.
We can't help you any further because we have no access or knowledge of what NewClass is.
subject: NewGUI is not abstract and does not override abstract method