improved the setUserData method.

jme3-core/src/main/java/com/jme3/scene/Spatial.java

@@ -1524,21 +1524,24 @@ public abstract class Spatial implements Savable, Cloneable, Collidable, Cloneab
     }
 
     public void setUserData(String key, Object data) {
-        if (userData == null) {
-            userData = new HashMap<String, Savable>();
-        }
-
         if (data == null) {
+            if (userData != null) {
                 userData.remove(key);
                 if(userData.isEmpty()) {
                     userData = null;
                 }
-        }else if (data instanceof Savable) {
+            }
+        } else {
+            if (userData == null) {
+                userData = new HashMap<String, Savable>();
+            }
+            if (data instanceof Savable) {
                 userData.put(key, (Savable) data);
             } else {
                 userData.put(key, new UserData(UserData.getObjectType(data), data));
             }
         }
+    }
 
     @SuppressWarnings("unchecked")
     public <T> T getUserData(String key) {