在API等级＆gt; = 21的设计模式（Android Studio 1.5.1）中查看我的布局时，出现以下问题：
java.lang.NullPointerException
at android.graphics.drawable.LayerDrawable$ChildDrawable.<init>(LayerDrawable.java:968)
at android.graphics.drawable.LayerDrawable$LayerState.<init>(LayerDrawable.java:1014)
at android.graphics.drawable.RippleDrawable$RippleState.<init>(RippleDrawable.java:910)
at android.graphics.drawable.RippleDrawable.createConstantState(RippleDrawable.java:901)
at android.graphics.drawable.RippleDrawable.createConstantState(RippleDrawable.java:90)
at android.graphics.drawable.LayerDrawable.mutate(LayerDrawable.java:932)
at android.graphics.drawable.RippleDrawable.mutate(RippleDrawable.java:891)
at android.view.View.applyBackgroundTint(View.java:16324)
at android.view.View.setBackgroundDrawable(View.java:16193)
at android.support.design.widget.FloatingActionButton.access$201(FloatingActionButton.java:58)
at android.support.design.widget.FloatingActionButton$1.setBackgroundDrawable(FloatingActionButton.java:144)
at android.support.design.widget.FloatingActionButtonLollipop.setBackgroundDrawable(FloatingActionButtonLollipop.java:71)
at android.support.design.widget.FloatingActionButton.<init>(FloatingActionButton.java:161)
at android.support.design.widget.FloatingActionButton.<init>(FloatingActionButton.java:104)
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:422)
at org.jetbrains.android.uipreview.ViewLoader.createNewInstance(ViewLoader.java:458)
at org.jetbrains.android.uipreview.ViewLoader.loadClass(ViewLoader.java:170)
at org.jetbrains.android.uipreview.ViewLoader.loadView(ViewLoader.java:103)
at com.android.tools.idea.rendering.LayoutlibCallbackImpl.loadView(LayoutlibCallbackImpl.java:168)
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:400)
at com.android.layoutlib.bridge.Bridge.createSession(Bridge.java:332)
at com.android.ide.common.rendering.LayoutLibrary.createSession(LayoutLibrary.java:350)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:510)
at com.android.tools.idea.rendering.RenderTask$2.compute(RenderTask.java:498)
at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:888)
at com.android.tools.idea.rendering.RenderTask.createRenderSession(RenderTask.java:498)
at com.android.tools.idea.rendering.RenderTask.access$600(RenderTask.java:72)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:610)
at com.android.tools.idea.rendering.RenderTask$3.call(RenderTask.java:607)
at com.android.tools.idea.rendering.RenderService.runRenderAction(RenderService.java:362)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:607)
at com.android.tools.idea.rendering.RenderTask.render(RenderTask.java:629)
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$1.run(Alarm.java:351)
at java.util.concurrent.Executors$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)
对于API级别19及以下，我没有收到此错误。
我使用以下依赖项：
compile 'com.android.support:appcompat-v7:23.1.1'
compile 'com.android.support:recyclerview-v7:23.1.1'
compile 'com.android.support:design:23.1.1'
compile 'com.google.android.gms:play-services-appindexing:8.4.0'
我工厂的XML代码是：
<android.support.design.widget.FloatingActionButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/fab"
android:fabSize="mini"
tools:backgroundTint="@color/fab_color_normal"
tools:rippleColor="@android:color/transparent"
tools:borderWidth="@dimen/fabBorderWidth"
android:layout_margin="@dimen/fab_compat_margin"
tools:elevation="6dp"
tools:pressedTranslationZ="12dp"
android:layout_gravity="bottom|right"
android:layout_marginRight="@dimen/fab_marginRight"
android:layout_marginBottom="@dimen/fab_marginBottom"
android:src="@mipmap/ic_play_arrow_white_24dp" />