title: No orientation specified, and the default is horizontal. This is a common source of bugs when children are added dynamically
body: <p>My application can not be run since there is an error in my xml-file. One of my LinearLayouts are marked red and by hovering I can see the following error message: 
  No orientation specified, and the default is horizontal. This is a common source of bugs when children are added dynamically.</p>
  
  <p>You can see my code below and I will mark the line were I get this message as follows: 
  &lt;- error message -></p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:orientation="horizontal"&gt;
  
      &lt;LinearLayout 
          android:layout_height="wrap_content"
          android:layout_width="0dp"
          android:layout_weight="1"
          android:orientation="vertical"
          &gt;
          &lt;Button 
              android:id="@+id/btn1"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="Fragment 1"
              /&gt;
  
          &lt;Button 
              android:id="@+id/btn2"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="Fragment 2"
              /&gt;
  
          &lt;Button 
              android:id="@+id/btn3"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="Fragment 3"
              /&gt;
  
      &lt;/LinearLayout&gt;
  
      &lt;LinearLayout             &lt;- error message -&gt; 
          android:id="@+id/myFragment"
          android:layout_width="0dp"
          android:layout_height="fill_parent"
          android:layout_weight="3" /&gt;
  
  
  &lt;/LinearLayout&gt;
  </code></pre>