title: unable to render facebook login button on android studio
body: <p>i'm facing a strange problem with facebook login button of facebook android sdk 4. 
  I've follow the guide to convert my previous code (i've used old facebook sdk), but android studio could not render correctly the button.
  this is my login button code:</p>
  
  <pre><code>&lt;com.facebook.login.widget.LoginButton
      xmlns:fb="http://schemas.android.com/apk/res-auto"
      android:id="@+id/fb_button"
      style="@style/FacebookLoginButton"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_above="@+id/my_button"
      android:layout_centerHorizontal="true"
      android:layout_marginBottom="-14dp"
      android:textSize="34sp"
      fb:login_text="@string/login_with_facebook"
      fb:logout_text="Logout" /&gt;
  </code></pre>
  
  <p>wbhere facebook login button style is</p>
  
  <pre><code> &lt;style name="FacebookLoginButton"&gt;
      &lt;item name="android:background"&gt;@drawable/button_facebook&lt;/item&gt;
      &lt;item name="android:textColor"&gt;@color/white&lt;/item&gt;
      &lt;item name="android:textSize"&gt;21sp&lt;/item&gt;
      &lt;item name="android:gravity"&gt;center&lt;/item&gt;
  </code></pre>
  
  <p>now when i try to see preview on android studio, i've this error:</p>
  
  <pre><code>java.lang.NoClassDefFoundError: Could not initialize class com.facebook.login.widget.LoginButton
  at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
  at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
  at java.lang.reflect.Constructor.newInstance(Constructor.java:408)
  at org.jetbrains.android.uipreview.ViewLoader.createNewInstance(ViewLoader.java:413)
  at org.jetbrains.android.uipreview.ViewLoader.loadView(ViewLoader.java:105)
  at com.android.tools.idea.rendering.LayoutlibCallback.loadView(LayoutlibCallback.java:176)
  at android.view.BridgeInflater.loadCustomView(BridgeInflater.java:207)
  at android.view.BridgeInflater.createViewFromTag(BridgeInflater.java:132)
  at android.view.LayoutInflater.rInflate_Original(LayoutInflater.java:806)
  at android.view.LayoutInflater_Delegate.rInflate(LayoutInflater_Delegate.java:64)
  at android.view.LayoutInflater.rInflate(LayoutInflater.java:782)
  at android.view.LayoutInflater.inflate(LayoutInflater.java:504)
  at android.view.LayoutInflater.inflate(LayoutInflater.java:385)
  at com.android.layoutlib.bridge.impl.RenderSessionImpl.inflate(RenderSessionImpl.java:410)
  at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:321)
  at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:350)
  at com.android.tools.idea.rendering.RenderService$5.compute(RenderService.java:708)
  at com.android.tools.idea.rendering.RenderService$5.compute(RenderService.java:697)
  at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:932)
  at com.android.tools.idea.rendering.RenderService.createRenderSession(RenderService.java:697)
  at com.android.tools.idea.rendering.RenderService.render(RenderService.java:816)
  at com.intellij.android.designer.designSurface.AndroidDesignerEditorPanel$6.run(AndroidDesignerEditorPanel.java:480)
  at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:320)
  at com.intellij.util.ui.update.MergingUpdateQueue.execute(MergingUpdateQueue.java:310)
  at com.intellij.util.ui.update.MergingUpdateQueue$2.run(MergingUpdateQueue.java:254)
  at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:269)
  at com.intellij.util.ui.update.MergingUpdateQueue.flush(MergingUpdateQueue.java:227)
  at com.intellij.util.ui.update.MergingUpdateQueue.run(MergingUpdateQueue.java:217)
  at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:238)
  at com.intellij.util.Alarm$Request$1.run(Alarm.java:327)
  at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
  at java.util.concurrent.FutureTask.run(FutureTask.java:266)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
  at java.lang.Thread.run(Thread.java:745)
  </code></pre>
  
  <p>what's wrong?</p>