Index: trunk/src/org/openstreetmap/josm/data/preferences/sources/SourcePrefHelper.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/preferences/sources/SourcePrefHelper.java	(revision 15214)
+++ trunk/src/org/openstreetmap/josm/data/preferences/sources/SourcePrefHelper.java	(revision 15215)
@@ -47,5 +47,5 @@
     public Map<String, String> serialize(SourceEntry entry) {
         Map<String, String> res = new HashMap<>();
-        res.put("url", entry.url);
+        res.put("url", entry.url == null ? "" : entry.url);
         res.put("title", entry.title == null ? "" : entry.title);
         return res;