title: Negative pixels in a layout margin
body: <p>I'm editing an open source project and find that a -1dp value for layout_marginTop is causing the graphical preview in Eclipse to fail with the error. The full error message, which probably includes an additional error, is:</p>
  
  <pre><code>java.lang.NullPointerException
  "-1dp" in attribute "layout_marginTop" is not a valid format.
  Exception details are logged in Window &gt; Show View &gt; Error LogThe graphics preview in the layout editor may not be accurate:
  Paint.setShadowLayer is not supported. (Ignore for this session)
  </code></pre>
  
  <p>Do you know what the problem is?</p>