fix null checks

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

@@ -603,6 +603,9 @@ public abstract class Spatial implements Savable, Cloneable, Collidable, Cloneab
      * @see MatParamOverride
      */
     public void addMatParamOverride(MatParamOverride override) {
+        if (override == null) {
+            throw new IllegalArgumentException("override cannot be null");
+        }
         localOverrides.add(override);
         setMatParamOverrideRefresh();
     }

jme3-core/src/main/java/com/jme3/shader/Uniform.java

@@ -195,7 +195,7 @@ public class Uniform extends ShaderVariable {
         }
 
         if (value == null) {
-            throw new NullPointerException();
+            throw new IllegalArgumentException("for uniform " + name + ": value cannot be null");
         }
 
         setByCurrentMaterial = true;