Merge change 24807 into eclair

* changes:
  Fix a drag and drop issue: if a drop happened too quickly it could go to the wrong screen.
diff --git a/src/com/android/launcher2/LauncherModel.java b/src/com/android/launcher2/LauncherModel.java
index b261a75..fae1d17 100644
--- a/src/com/android/launcher2/LauncherModel.java
+++ b/src/com/android/launcher2/LauncherModel.java
@@ -310,7 +310,7 @@
                 mAllAppsList.modified = new ArrayList();
             }
 
-            final Callbacks callbacks = mCallbacks.get();
+            final Callbacks callbacks = mCallbacks != null ? mCallbacks.get() : null;
             if (callbacks == null) {
                 return;
             }