title: Android Studio shows red exclamation at drawable attribute
body: <p>In layout XML files, the red exclamation mark is displayed left of <code>@drawable</code> references. Almost all <code>drawable</code>s have this mark:</p>
  
  <p><a href="https://i.stack.imgur.com/8fy8w.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8fy8w.png" alt="enter image description here"></a></p>
  
  <p>The content of <code>shape_detail_btn_border.xml</code> is as below. It is a group of shapes and does not have any broken image resource:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;shape xmlns:android="http://schemas.android.com/apk/res/android"
  android:shape="rectangle"&gt;
      &lt;solid android:color="@android:color/white"/&gt;
      &lt;corners android:radius="5dp"/&gt;
      &lt;stroke android:width="1dp" android:color="@color/bapul_color_d5d5d5"/&gt;
  &lt;/shape&gt;
  </code></pre>
  
  <p>I hover the mouse on the mark and nothing shows up. what does this mean?</p>
  
  <p>Note that the Android Studio version is 1.4 (AI-141.2288178), currently the most recent version. And, there's <strong>no problem</strong> to build and run the app at all.</p>