Nie masz w xml'u activity_main obiektu FloatingActionButton. Jeśli nie jest Ci potrzebny, to usuń w klasie MainActivity ten fragment kodu:
FloatingActionButton fab = findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});