Android - Actionbar not showing in the development preview

If you can't see the action bar AND also the status bar or buttons, it could be because the "Show Layout Decorations" options is turned off in the Layout Editor. Just check this option and everything should be displayed again. As per Android Studio 3.3:

Check which theme is selected on the top of the XML preview screen.

Even if you're using Eclipse, it will look similar.

Your application is displaying the ActionBar because your AndroidManifest has a theme which displays the ActionBar.