尝试根据空对象引用期望调用虚拟方法'void android.widget.TextView.setText（java.lang.CharSequence）'

Question

我正在制作一个能够逐行读取文本文件的应用程序。我的应用程序中有一些TextView，它们从这些行中获取文本，但是我遇到了一个期望：

Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference

这是我的代码：

    public File file = new File("/sdcard/datacuaapp.txt");
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            checkQuyen();
            dong[0]="";
            dong[1]="";
            dong[2]="";
            dong[3]="";
            TextView tv1=(TextView) findViewById(R.id.so1);
            TextView tv2=(TextView) findViewById(R.id.so2);
            TextView tv3=(TextView) findViewById(R.id.so3);
            TextView tv4=(TextView) findViewById(R.id.so4);
            if (!file.exists())
            {
                try {
                    FileWriter writer = new FileWriter(file);
                    writer.append("@" + "\n" + "@" + "\n" + "@" + "\n" + "@" + "\n" );
                    writer.flush();
                    writer.close();
                }
                catch(IOException io)
                {
                    io.printStackTrace();

                }
            }
            else{
                doc();  // the problem is in these lines
                tv1.setText(dong[0]);
                tv2.setText(dong[1]);
                tv3.setText(dong[2]);
                tv4.setText(dong[3]);
            }
 }

    public String[] dong = new String[4];

    public void doc()
    {
        Integer a = 0;
        try {
            FileReader fileReader = new FileReader(file);
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            StringBuffer stringBuffer = new StringBuffer();
            String line=bufferedReader.readLine();
            while (a<4) {
                stringBuffer.append(line);
                stringBuffer.append("\n");
                dong[a]=line;
                a++;
            }
            fileReader.close();
        }
        catch(IOException e)
        {
            e.printStackTrace();
        }

    }