Index: /trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java	(revision 7049)
+++ /trunk/src/org/openstreetmap/josm/gui/dialogs/relation/GenericRelationEditor.java	(revision 7050)
@@ -83,4 +83,5 @@
 import org.openstreetmap.josm.gui.tagging.ac.AutoCompletingTextField;
 import org.openstreetmap.josm.gui.tagging.ac.AutoCompletionList;
+import org.openstreetmap.josm.tools.CheckParameterUtil;
 import org.openstreetmap.josm.tools.ImageProvider;
 import org.openstreetmap.josm.tools.Shortcut;
@@ -736,5 +737,6 @@
     }
 
-    public static Command addPrimitivesToRelation(final Relation orig, Collection<? extends OsmPrimitive> primitivesToAdd) {
+    public static Command addPrimitivesToRelation(final Relation orig, Collection<? extends OsmPrimitive> primitivesToAdd) throws IllegalArgumentException {
+        CheckParameterUtil.ensureParameterNotNull(orig, "orig");
         try {
             final Collection<TaggingPreset> presets = TaggingPreset.getMatchingPresets(EnumSet.of(TaggingPresetType.RELATION), orig.getKeys(), false);
@@ -742,5 +744,5 @@
             boolean modified = false;
             for (OsmPrimitive p : primitivesToAdd) {
-                if (p instanceof Relation && orig != null && orig.equals(p)) {
+                if (p instanceof Relation && orig.equals(p)) {
                     warnOfCircularReferences(p);
                     continue;
Index: /trunk/src/org/openstreetmap/josm/io/OsmServerBackreferenceReader.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/OsmServerBackreferenceReader.java	(revision 7049)
+++ /trunk/src/org/openstreetmap/josm/io/OsmServerBackreferenceReader.java	(revision 7050)
@@ -264,6 +264,6 @@
             visitor.merge();
             ret = visitor.getTargetDataSet();
-            readIncompletePrimitives(ret, progressMonitor.createSubTaskMonitor(1, false));
             if (ret != null) {
+                readIncompletePrimitives(ret, progressMonitor.createSubTaskMonitor(1, false));
                 ret.deleteInvisible();
             }