我正尝试从一个 REST API 获取一些数据，但我遇到了一个问题，因为我有一些包含不同数量项的对象。
例如：
    [{7 项}, {7 项}, {6 项}...{1 项}]
它们有相同的键，但问题在于在某些对象中，一个或多个参数可能会丢失。
我的应用程序开始运行，但当我尝试向下滚动时，它会崩溃。我收到了以下错误：
    尝试从空对象引用中读取字段 'android.widget.TextView com.example.covidapp.covidAdapter$ViewHolder.tv_activeCases'。
这是我的 `MainActivity` 和 `ListView`：
    public class MainActivity extends AppCompatActivity {
        private static final String TAG = "MainActivity";
        private ListView listView;
        private covidAdapter covidAdapter;
        private CovidApi covidApi;
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    
            listView = (ListView) findViewById(R.id.list_view_main);
    
            Gson gson = new GsonBuilder().serializeNulls().create();
    
            Retrofit retrofit = new Retrofit.Builder().baseUrl("https://covid-19-tracking.p.rapidapi.com/")
                    .addConverterFactory(GsonConverterFactory.create(gson)).build();
    
            covidApi = retrofit.create(CovidApi.class);
    
            Call<List<covid>> call = covidApi.getInfo();
    
                call.enqueue(new Callback<List<covid>>() {
    
                @Override
                public void onResponse(Call<List<covid>> call, Response<List<covid>> response) {
    
                    if (!response.isSuccessful()){
                        return;
                    }
    
                    List<covid> covids = response.body();
    
    
                    covidAdapter = new covidAdapter((ArrayList<covid>) covids, getApplicationContext());
                    listView.setAdapter(covidAdapter);
    
                }
    
                @Override
                public void onFailure(Call<List<covid>> call, Throwable t) {
                    Toast.makeText(MainActivity.this,t.getMessage(),Toast.LENGTH_LONG).show();
                }
            });
        }
    }
这是 `Adapter` 类：
    public class covidAdapter extends BaseAdapter {
    
        private ArrayList<covid> covidList;
        private Context context;
    
        public covidAdapter(ArrayList<covid> covidList, Context context){
            this.covidList = covidList;
            this.context = context;
        }
    
        @Override
        public int getCount() {
            return this.covidList.size();
        }
    
        @Override
        public Object getItem(int i) {
            return this.covidList.get(i);
        }
    
        @Override
        public long getItemId(int i) {
            return i;
        }
    
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
    
            ViewHolder holder = null;
    
            if (convertView == null) {
    
                LayoutInflater inf = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                convertView = inf.inflate(R.layout.template, null);
    
                holder = new ViewHolder();
    
                holder.tv_activeCases =(TextView)convertView.findViewById(R.id.textview_active_cases);
                holder.tv_country =(TextView)convertView.findViewById(R.id.textview_country);
                //holder.tv_lastUpdate =(TextView)convertView.findViewById(R.id.textview_last_update);
                holder.tv_newCases =(TextView)convertView.findViewById(R.id.textview_new_cases);
                holder.tv_newDeaths =(TextView)convertView.findViewById(R.id.textview_new_deaths);
                holder.tv_totalCases =(TextView)convertView.findViewById(R.id.textview_total_cases);
                holder.tv_totalDeaths =(TextView)convertView.findViewById(R.id.textview_total_deaths);
                holder.tv_totalRecovered =(TextView)convertView.findViewById(R.id.textview_total_recovered);
    
    
            } else {
                holder = (ViewHolder) convertView.getTag();
            }
    
            covid covid = covidList.get(position);
    
            if ( covidList.size() < 0){
                System.out.println("No dps key");
    
            }else {
    
                holder.tv_activeCases.setText("Active Cases: " + covid.getActive());
                holder.tv_country.setText("Country: " + covid.getCountry());
                // holder.tv_lastUpdate.setText("Last Update: " + covid.getLastUpdate());
                holder.tv_newDeaths.setText("New Deaths: " + covid.getNewDeaths());
                holder.tv_newCases.setText("New Cases: " + covid.getNewCases());
                holder.tv_totalRecovered.setText("Recovered: " + covid.getRecovered());
                holder.tv_totalDeaths.setText("Deaths: " + covid.getTotalDeaths());
                holder.tv_totalCases.setText("Total Cases: " + covid.getTotalCases());
            }
            return convertView;
    
        }
    
    
        private static class ViewHolder{
            public TextView tv_country, tv_activeCases,  tv_newCases, tv_newDeaths, tv_totalCases, tv_totalDeaths, tv_totalRecovered;
        }
    }
我将不胜感激任何帮助。

convertView.setTag(holder)