Android AlertDialog不可见

Question

当调用alertDialog.show时，它会弹出一个对话框，但它仍然不可见。我必须点击屏幕，直到我碰巧点击警报上的“确定”按钮，即使它没有明显显示。

public class RecordSignals extends Activity {

    private ImageView pulseOxImage;
    static AlertDialog poDialog;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(saveInstanceState);

        pulseOxImage = new ImageView(this);
        pulseOxImage.setImageDrawable(ContextCompat
                .getDrawable(this,R.drawable.pulse_ox_animation));

        poDialog = new AlertDialog.Builder(this).create();

        ...
    }

    final Runnable callback = new Runnable() {
        @Override
        public void run() {
            Thread thread = new Thread() {
                @Override
                public void run() {
                    ...
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            if (pulseOxArtifact >= 2 || pulseOxOutOfTrack >= 2 || pulseOxSensorAlarm >= 2) {
                                pulseOxStatus();
                                pulseOxImage.setVisibility(View.VISIBLE);
                                poDialog.show();
                            }
                        }
                     });
                 }
             };
         }
    };

    public void pulseOxStatus() {
        poDialog.setView(pulseOxImage);
        poDialog.setButton("OK", new DialogInterface.OnClicklistener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                poDialog.dismiss();
            }
        });
    }
}