null pointer exception.

package com.jordanernst.golfscorecard;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.TextView;

/**
 * Created by insan_000 on 2016-03-21.
 */
public class ListAdapter extends BaseAdapter {
    private final Hole[] mHoles;
    private final Context mContext;

    public ListAdapter(Context context, Hole[] holes){
        mContext= context;
        mHoles = holes;
    }


    @Override
    public int getCount() {
        return mHoles.length;
    }

    @Override
    public Object getItem(int position) {
        return mHoles[position];
    }

    @Override
    public long getItemId(int position) {
        return 0;// not implemented
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        final ViewHolder holder;

        if(convertView == null){
            convertView= LayoutInflater.from(mContext).inflate(R.layout.list_item, null);// ibelieve passing null here is wrong

            holder= new ViewHolder();
            holder.holeLabel = (TextView) convertView.findViewById(R.id.holeLabel);
            holder.strokeCount = (TextView) convertView.findViewById(R.id.strokeCount);
            holder.removeStrokeButton = (Button) convertView.findViewById(R.id.removeStrokeButton);
            holder.addStrokeButton = (Button) convertView.findViewById(R.id.addStrokeButton);

            convertView.setTag(holder);
        }
        else{
            holder = (ViewHolder) convertView.getTag();

        }
        holder.holeLabel.setText(mHoles[position].getLabel());
        holder.strokeCount.setText(mHoles[position].getStrokeCount());
        holder.removeStrokeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int updatedStrokeCount = mHoles[position].getStrokeCount() - 1;
                if (updatedStrokeCount < 0) updatedStrokeCount = 0;
                mHoles[position].setStrokeCount(updatedStrokeCount);
                holder.strokeCount.setText(updatedStrokeCount + "");
            }
        });
        holder.addStrokeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int updatedStrokeCount =  mHoles[position].getStrokeCount() + 1;
                mHoles[position].setStrokeCount(updatedStrokeCount);
                holder.strokeCount.setText(updatedStrokeCount + "");
            }
        });


        return convertView;
    }

    private static class ViewHolder{
        TextView holeLabel;
        TextView strokeCount;
        Button addStrokeButton;
        Button removeStrokeButton;
    }

}