game crash after inserting a loop for reset game

I'm trying to create a connect style game for an assignment. Everything went well until I added the code for the playAgain layout which is supposed to reset the game to the start, but instead it crashes. I keep getting an error for the gridlayout for some reason.

public class MainActivity extends AppCompatActivity {
    //yellow=0 ,red=1
    //2=not played
    int activePlayer=0;
    int[]gameState={2,2,2,2,2,2,2,2,2,};
    int[][] winningPositions={{0,1,2},{3,4,5},{6,7,8},{0,3,6},
            {1,4,7},{2,5,8},{0,4,8},{2,4,6}};


    public void dropIn (View view){
        ImageView counter=(ImageView) view;


        int tappedCounter=Integer.parseInt(counter.getTag().toString());
        if (gameState[tappedCounter]==2) {
            gameState[tappedCounter]=activePlayer;

            counter.setTranslationY(-1000f);
            if (activePlayer == 0) {
                counter.setImageResource(R.drawable.yellow);
                activePlayer = 1;

            } else {
                counter.setImageResource(R.drawable.red);
                activePlayer = 0;

            }


            counter.animate().translationYBy(1000f).setDuration(300);

            for (int[] winningPositions : winningPositions){
                if (gameState[winningPositions[0]]==gameState[winningPositions[1]]
                        && gameState[winningPositions[1]]==gameState[winningPositions[2]] &&
                 gameState[winningPositions[0]]!=2){
                    System.out.println(gameState[winningPositions[0]]);

                    String winner ="Red";

                    if (gameState[winningPositions[0]]==0){

                        winner="Yellow";
                    }




                    //someone has won

                    TextView winnerMessage=(TextView) findViewById(R.id.winnerMessage);
                    winnerMessage.setText(winner + " has WON!");
                    LinearLayout layout = (LinearLayout)findViewById(R.id.playAgainLayout);
                    layout.setVisibility(View.VISIBLE);
                }



            }

        }





    }

    public void playAgain(View view) {


        LinearLayout layout = (LinearLayout)findViewById(R.id.playAgainLayout);
        layout.setVisibility(View.INVISIBLE);

        activePlayer=0;

        for (int i= 0;i<gameState.length;i++){

            gameState [i]=2;
        }


             GridLayout gridLayout=(GridLayout)findViewById(R.id.gridLayout);

        for (int i=0;i<gridLayout.getChildCount();i++){
            ((ImageView) gridLayout.getChildAt(i)).setImageResource(0);

        }

    }


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}