Bug 888665 - Bad stored add-ons cause document generation failure in the presence of good add-ons. r=nalexander

mobile/android/base/background/healthreport/HealthReportGenerator.java

@@ -427,7 +427,16 @@ public class HealthReportGenerator {
 
   /**
    * Compute the *tree* difference set between the two objects. If the two
-   * objects are identical, returns null.
+   * objects are identical, returns <code>null</code>. If <code>from</code> is
+   * <code>null</code>, returns <code>to</code>. If <code>to</code> is
+   * <code>null</code>, behaves as if <code>to</code> were an empty object.
+   *
+   * (Note that this method does not check for {@link JSONObject#NULL}, because
+   * by definition it can't be provided as input to this method.)
+   *
+   * This behavior is intended to simplify life for callers: a missing object
+   * can be viewed as (and behaves as) an empty map, to a useful extent, rather
+   * than throwing an exception.
    *
    * @param from
    *          a JSONObject.
@@ -445,10 +454,14 @@ public class HealthReportGenerator {
   public static JSONObject diff(JSONObject from,
                                 JSONObject to,
                                 boolean includeNull) throws JSONException {
-    if (from == null || from == JSONObject.NULL) {
+    if (from == null) {
       return to;
     }
 
+    if (to == null) {
+      return diff(from, new JSONObject(), includeNull);
+    }
+
     JSONObject out = new JSONObject();
 
     HashSet<String> toKeys = includeNull ? new HashSet<String>(to.length())