Cannot resolve symbole 'R' the the whole project

Question

I tried to add a new activity to my project which produced this error:

Cannot resolve symbole 'R'

I got it in ALL of my previous activities too for no reason

Here is an example:

@SuppressLint("SimpleDateFormat") 
public class Startbildschirm extends ActionBarActivity
        {

            final int PIC_CROP = 2;
            static final int REQUEST_TAKE_PHOTO = 1;

            @Override
            protected void onCreate(Bundle savedInstanceState)
            {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.activity_startbildschirm);
                System.loadLibrary( Core.NATIVE_LIBRARY_NAME );
            }


            @Override
            public boolean onCreateOptionsMenu(Menu menu)
            {
                // Inflate the menu; this adds items to the action bar if it is present.
                getMenuInflater().inflate(R.menu.startbildschirm, menu);
                return true;
            }

What I've done to try to resolve the issue:

• Clean project
• Rebuild Project
• Restart program and computer
• Sync projects with gradle files

I've also read a lot of topics about it but non of the solutions worked out.

When I try to run the program I get this error:

Execution failed for task :formelerkennung:processDebugManifest. java.lang.NullPointerException (no error message)