Androidアプリがボタンをクリックするだけで落ちてしまうてしまう

このようなエラーが出ます。どこがおかしいのでしょうか。

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

以下がコードとなっています。

package com.example.tokoroshingo.myapplication;
import android.content.Intent;
import android.content.res.Resources;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.content.Context;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
public class FileActivity1 extends AppCompatActivity {
    private TextView textView;
    private EditText editText;
    private String fileName[] = new String[15] ;
    private int i;
    private Button buttonRead[] = new Button[15];

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file1);

        textView = findViewById(R.id.text_view);

        for(i=0; i<15; i++){
            fileName[i] = "CH1_"+i+1+".txt";
            int viewId;
            String resViewName;
            resViewName = "button_ch" + i+1;
            viewId = getResources().getIdentifier(resViewName, "id", getPackageName());
            buttonRead[i] = findViewById(viewId);
            buttonRead[i].setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    String str = readFile(fileName[i]);
                    if (str != null) {
                        textView.setText(str);
                    } else {
                        textView.setText(R.string.read_error);
                    }
                    TextView textReceive1 = (TextView) findViewById(R.id.text_view1);
                    textReceive1.setText("CH"+ i);
                }
            });

        }

        //  editText = findViewById(R.id.edit_text);

        Button return_file_Button1 = findViewById(R.id.return_file_button1);
        return_file_Button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(getApplication(), SubActivity5.class);
                startActivity(intent);
            }
        });

    }

    // ファイルを読み出し
    public String readFile(String file) {
        String text = null;

        try {
            FileInputStream in = openFileInput(file);
            BufferedReader reader = new BufferedReader(new InputStreamReader(in, "UTF-8"));
            String str = "";
            String tmp;
            while ((tmp = reader.readLine()) != null) {
                str = str + tmp + "\n";
            }
            text = str;
            reader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return text;
    }
}