SkeletonControl: fix syntax error

Conflicts:
	jme3-core/src/main/java/com/jme3/animation/SkeletonControl.java

jme3-core/src/main/java/com/jme3/animation/SkeletonControl.java

@@ -111,7 +111,7 @@ public class SkeletonControl extends AbstractControl implements Cloneable, JmeCl
      * Material references used for hardware skinning
      */
     private Set<Material> materials = new HashSet<Material>();
-
+    
     /**
      * Serialization only. Do not use.
      */
@@ -204,6 +204,9 @@ public class SkeletonControl extends AbstractControl implements Cloneable, JmeCl
      * @param skeleton the skeleton
      */
     public SkeletonControl(Skeleton skeleton) {
+        if (skeleton == null) {
+            throw new IllegalArgumentException("skeleton cannot be null");
+        }
         this.skeleton = skeleton;
     }