title: Android: "Error parsing XML: must not undeclare prefix"
body: <p>I am very new to android development and was in the process of trying to create my first process when I came across the build error with which the question is titled.</p>
  
  <p>Here is my XML code:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:app= ""
       xmlns:tools="http://schemas.android.com/tools"
       android:id="@+id/activity_main"
       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"
       tools:context="com.example.austinc.clicker.MainActivity"&gt;
  
       &lt;TextView
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="0"
           android:id="@+id/textViewCount"
           android:layout_alignParentTop="true"
           android:layout_centerHorizontal="true"
           android:textSize="24dp" /&gt;
  
       &lt;Button
           android:text="Reset"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:layout_below="@+id/ButtonClick"
           android:layout_alignLeft="@+id/ButtonClick"
           android:layout_alignStart="@+id/ButtonClick"
           android:layout_marginTop="61dp"
           android:id="@+id/buttonReset" /&gt;
  
       &lt;Button
           android:text="Click"
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:layout_marginTop="55dp"
           android:id="@+id/ButtonClick"
           android:layout_below="@+id/textViewCount"
           android:layout_centerHorizontal="true" /&gt;
  &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>From what I can infer by double clicking on the error, the problem has to do with the second line of code (the meaning of which I have no idea). Is this a quick fix, or have I made a mistake somewhere in editing some piece of code?</p>