Allow null to be re-bound as a root script.  This was always legal from native and just a limitation of the java binding for the API.
diff --git a/graphics/java/android/renderscript/RenderScript.java b/graphics/java/android/renderscript/RenderScript.java
index 076a5d4..1bdabe7 100644
--- a/graphics/java/android/renderscript/RenderScript.java
+++ b/graphics/java/android/renderscript/RenderScript.java
@@ -281,7 +281,11 @@
     // Root state
 
     public void contextBindRootScript(Script s) {
-        nContextBindRootScript(s.mID);
+        int id = 0;
+        if(s != null) {
+            id = s.mID;
+        }
+        nContextBindRootScript(id);
     }
 
     //public void contextBindSampler(Sampler s, int slot) {