Fixed IllegalArgumentException when removing a LodControl from a spatial

LodControl throws an exception when the spatial being added to isn't a geometry, however, when the controls is removed it calls setSpatial with a null value triggering the exception throw. Now checking for this null value case.

jme3-core/src/main/java/com/jme3/scene/control/LodControl.java

@@ -121,11 +121,13 @@ public class LodControl extends AbstractControl implements Cloneable, JmeCloneab
 
     @Override
     public void setSpatial(Spatial spatial) {
-        if (!(spatial instanceof Geometry)) {
+        if (spatial != null && !(spatial instanceof Geometry)) {
             throw new IllegalArgumentException("LodControl can only be attached to Geometry!");
         }
 
         super.setSpatial(spatial);
+        
+        if(spatial != null) {
             Geometry geom = (Geometry) spatial;
             Mesh mesh = geom.getMesh();
             numLevels = mesh.getNumLodLevels();
@@ -133,6 +135,10 @@ public class LodControl extends AbstractControl implements Cloneable, JmeCloneab
             for (int i = numLevels - 1; i >= 0; i--) {
                 numTris[i] = mesh.getTriangleCount(i);
             }
+        } else {
+            numLevels = 0;
+            numTris = null;
+        }
     }
 
     @Override