ProgressDialog:如何防止泄漏窗口

public class MainActivity extends AppCompatActivity {
    private ProgressDialog progressDialog;

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

        progressDialog = new ProgressDialog(this);
        progressDialog.setMessage("Loading...");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        if (progressDialog != null) {
            progressDialog.dismiss();
            progressDialog = null;
        }
    }
}

if (!isFinishing()) {
    progressDialog.show();
}

progressDialog.dismiss();

private class LoadDataTask extends AsyncTask<Void, Void, Void> {
    @Override
    protected Void doInBackground(Void... params) {
        // Load data here
    }

    @Override
    protected void onPostExecute(Void result) {
        progressDialog.dismiss();
    }
}