Android не могу привязать адаптер к GridView

При нажатии на startButton - GridView пустой. Где ошибка?

<EditText
    android:id="@+id/peopleCnt"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:width="60dp"
    android:text="1"
    android:gravity="center_horizontal" />

<GridView
    android:id="@+id/countersGrid"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:numColumns="3"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent">
</GridView>

list item

public class cnterItem {
    int num;
    int mils;
    boolean running;

    cnterItem(int num_){
        num=num_;
        mils=0;
        running=false;
    }
}

cnterItem.xml

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:id="@+id/itemContainer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <TextView
            android:id="@+id/num"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="#000000"
            />

        <TextView
            android:id="@+id/cnter"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="#000000"
            android:text="00:00:0" />
    </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

adapter

public class gridAdapter extends ArrayAdapter<cnterItem>{
    ArrayList<cnterItem> objects;

    public gridAdapter(Context ctx, ArrayList<cnterItem> items){
        super(ctx,0);
        objects=items;
    }

    @Override
    public cnterItem getItem(int position) {
        return objects.get(position);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent){
        final cnterItem citem= getItem(position);
        if (convertView == null) {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.cnter_item, parent, false);
        }
        LinearLayout l=convertView.findViewById(R.id.itemContainer);
        TextView num=convertView.findViewById(R.id.num);
        TextView cnter= convertView.findViewById((R.id.cnter));

        if(citem != null){
            num.setText(citem.num);
            int mins=(int)Math.floor(citem.mils/600);
            int secs=(int)Math.floor((citem.mils-mins*600)/10);
            int mils=citem.mils-(mins*600+secs*10);

            String m="",s="";
            if(mins<10) m="0"+mins;
            if(secs<10) s="0"+secs;

            cnter.setText(m+":"+s+":"+mils);
        }
        return convertView;
    }
}

startButton.setOnClickListener(new View.OnClickListener(){
    @Override
    public void onClick(View v){
        int pCnt=Integer.parseInt(peopleCnt.getText().toString());

        for(int i=0;i<pCnt;i++){
            cnterItems.add(new cnterItem(i+1));
        }
        gridAdapter adapter=new gridAdapter(getApplicationContext(),cnterItems);
        countersGrid.setAdapter(adapter);
    }
});