安卓app开发中运行过程中出现闪退的问题，请问怎么解决？

就是在运行过程中，点击发送按钮的时候就闪退了。

报错情况大概是这样：
E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #2
Process: com.example.tulingdemo, PID: 5775
java.lang.RuntimeException: An error occurred while executing doInBackground()
at android.os.AsyncTask$4.done(AsyncTask.java:415)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)...

代码如下：
public class MainActivity extends AppCompatActivity implements HttpGetDataListener, View.OnClickListener {

private HttpDate httpDate;
private List<ListData> lists;
private ListView lv;
private EditText sendtext;
private Button send_btn;
private String content_str;
private TextAdapter adapter;
private String[] welcome_array;
private double currentTime,oldtime = 0;


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

private void initView(){
    lv = (ListView) findViewById(R.id.lv);
    sendtext = (EditText) findViewById(R.id.sendText);
    send_btn = (Button) findViewById(R.id.send_btn);
    lists = new ArrayList<ListData>();
    send_btn.setOnClickListener((View.OnClickListener) this);
    adapter = new TextAdapter(lists,this);
    lv.setAdapter(adapter);
    ListData listData;
    listData = new ListData(getRandomWelcomeTips(),ListData.RESEIVE,getTime());
    lists.add(listData);
}

private String getRandomWelcomeTips(){
    String welcome_tip = null;
    welcome_array = this.getResources().getStringArray(R.array.welcome_tips);
    int index = (int) (Math.random()*(welcome_array.length -1));
    welcome_tip = welcome_array[index];
    return welcome_tip;

}
@Override
public void getDataUrl(String data){
    //System.out.println(data);
    parseText(data);
}

public  void  parseText(String data){
    try {
        JSONObject jb = new JSONObject();
        ListData listData;
        listData = new ListData(jb.getString("text"),ListData.RESEIVE,getTime());
        lists.add(listData);
        adapter.notifyDataSetChanged();
    }catch (JSONException e){
        e.printStackTrace();
    }

}


public void onClick(View v){
    getTime();
    content_str = sendtext.getText().toString();
    sendtext.setText("");
    String dropk = content_str.replace(" ","");
    String droph = dropk.replace("\n","");
    ListData listData;
    listData = new ListData(content_str,ListData.SEND,getTime());
    lists.add(listData);
    if(lists.size()>30){
        for(int i=0;i<lists.size();i++){
            lists.remove(i);
        }
    }
    adapter.notifyDataSetChanged();
    httpDate = (HttpDate) new HttpDate("http://api.qingyunke.com/api.php?key=free&appid=0&msg=你好"+ content_str,this).execute();
}

private   String getTime(){
    currentTime = System.currentTimeMillis();
    SimpleDateFormat format = new SimpleDateFormat("yyyy年MM月dd日 HH:mm:ss");
    Date curDate = new Date();
    String str = format.format(curDate);
    if(currentTime - oldtime >= 5*60*1000){
        oldtime = currentTime;
        return str;
    }else {
        return "";
    }
}

}

帮忙看看问题在哪吧！

集合是不能用for循环来遍历删除的，可以用迭代器。而且你既然每个下标都删除，为什么不用.clear()？要一个一个删？