To construct a StatefulWidget, create two classes: A
StatefulWidget and a corresponding State class.
Pro tip: The Flutter plugins for Android Studio and VSCode include
the stful snippet to quickly generate this code.

3. Display a button that toggles the visibility

Now that you have some data to determine whether the green box
should be visible, you need a way to update that data.
In this example, if the box is visible, hide it.
If the box is hidden, show it.

To handle this, display a button. When a user presses the button,
flip the boolean from true to false, or false to true.
Make this change using setState(),
which is a method on the State class.
This tells Flutter to rebuild the widget.

For more information on working with user input,
see the Gestures section of the cookbook.

4. Fade the box in and out