Admob Ad Banner reduilding the entire screen when it refreshes

Question

I have recently added admob test ad banner in my app,but it rebuilding the whole screen when it refresh to load new impression.I used a largeBanner as AdSize Here is my code.

  BannerAd _bannerAd;
  bool _isBannerAdReady = false;

  @override
  void initState() {
    super.initState();
    _bannerAd = BannerAd(
        size: AdSize.largeBanner,
        adUnitId: AdHelper.bannerAdUnitID,
        listener: BannerAdListener(
          onAdLoaded: (_){
            setState(() {
              _isBannerAdReady = true;
            });
          },
          onAdFailedToLoad: (ad,err){
            print("Error: ${err.message}");
            _isBannerAdReady = false;
            ad.dispose();
          }
        ),
        request: AdRequest()
    );
    _bannerAd.load();
  }

  @override
  void dispose() {
    super.dispose();
    _bannerAd.dispose();
  }

Ad Widget:

Column(
              children: [
                Container(
                  height: _bannerAd.size.height.toDouble(),
                  width: _bannerAd.size.width.toDouble(),
                  child: AdWidget(ad: _bannerAd,),
                ),
                SizedBox(height: 50.0,)
              ],
            )