Fix crash

app/src/main/res/layout/activity_file.xml

@@ -2,6 +2,7 @@
 <android.support.design.widget.CoordinatorLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
+    android:id="@+id/root"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     tools:context=".FileActivity" >