Admob have a bug?

My adMOB test app compile without problems (apk is created), but when
I try run it in my Samsung cellphone (SM-G531BT - Android 5.1.1) 
nothing is displayed, only the message: "The adMobTeste stopped." (an 
AlertDialog).

I used try...catch but it not display nothing.

In MainActivity.java:

package net.testes.admobteste;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdView;
public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    AlertDialog msg;
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    try {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        AdView propaganda = (AdView) findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder()
                .addTestDevice("xxE1Axx75481630A10256ED92AF3xxxx")
                .build();
        propaganda.loadAd(adRequest);
    } catch (Exception e) {
        //e.printStackTrace();
        //e.getMessage();
        builder.setTitle("Erro");
        builder.setMessage(e.getMessage());
        msg = builder.create();
        //Exibe
        msg.show();

    }
}
}

In strings.xml:

<resources>
<string name="app_name">AdMobTeste</string>
<string name="banner_ad_unit_id">ca-app-
      pub-3940256099942544/6300978111</string>
</resources>

In activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<com.google.android.gms.ads.AdView xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:visibility="visible"
    ads:adSize="BANNER"
    ads:adUnitId="@string/banner_ad_unit_id">
</com.google.android.gms.ads.AdView>
</RelativeLayout>

I'm testing banner use in android apps. Someone code doesn't display
here (xml and html).