Don't try to toggle next button state when the fragment isn't attached

b/12886252

Change-Id: Ie19d8e946139d317f7e867449141dcfd82c510f8
diff --git a/src/com/android/email/activity/setup/AccountSetupNamesFragment.java b/src/com/android/email/activity/setup/AccountSetupNamesFragment.java
index 490be6e..393f4c2 100644
--- a/src/com/android/email/activity/setup/AccountSetupNamesFragment.java
+++ b/src/com/android/email/activity/setup/AccountSetupNamesFragment.java
@@ -159,7 +159,10 @@
             }
         }
         final AccountSetupFinal activity = (AccountSetupFinal) getActivity();
-        activity.setNextButtonEnabled(enableNextButton);
+        if (activity != null) {
+            // If we're not attached to the activity, this state probably doesn't need updating
+            activity.setNextButtonEnabled(enableNextButton);
+        }
     }
 
     public String getDescription() {