title: Android: NumberPicker doesn´t work
body: <p>I added a NumberPicker to my layout and debugged my app. Now I see that the NumberPicker doesn´t work, I can´t see any "+" or "-" button and also when I click on a number on my keyboard, nothing happens. </p>
  
  <p>Here´s my layout file: </p>
  
  <pre><code>&lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  android:background="#99cc00"
  tools:context=".Gewichtsentwicklung" &gt;
  
  &lt;NumberPicker
      android:id="@+id/numberPicker1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentTop="true"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="80dp" /&gt;
  
  &lt;TextView
      android:id="@+id/tvGewichtUeberschrift"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentLeft="true"
      android:layout_alignParentRight="true"
      android:layout_centerHorizontal="true"
      android:textColor="@color/white"
      android:textSize="13pt"
      android:layout_alignParentTop="true"
      android:text="Aktuelles Gewicht eingeben" /&gt;
  
  &lt;Button
      android:id="@+id/btGewicht"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentLeft="true"
      android:layout_alignParentRight="true"
      android:layout_below="@+id/numberPicker1"
      android:layout_marginTop="79dp"
      android:text="Gewicht speichern" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>Where´s the problem?</p>