Index: trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java	(revision 15722)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/mapcss/Functions.java	(revision 15723)
@@ -11,4 +11,5 @@
 import java.util.Locale;
 import java.util.Map.Entry;
+import java.util.Objects;
 import java.util.TreeSet;
 import java.util.regex.Matcher;
@@ -311,5 +312,8 @@
     @NullableArguments
     public static String concat(Object... args) { // NO_UCD (unused code)
-        return Arrays.stream(args).map(String::valueOf).collect(Collectors.joining());
+        return Arrays.stream(args)
+                .filter(Objects::nonNull)
+                .map(String::valueOf)
+                .collect(Collectors.joining());
     }
 
Index: trunk/test/unit/org/openstreetmap/josm/plugins/PluginHandlerTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/plugins/PluginHandlerTest.java	(revision 15722)
+++ trunk/test/unit/org/openstreetmap/josm/plugins/PluginHandlerTest.java	(revision 15723)
@@ -71,7 +71,7 @@
         try {
             final String plugins = Stream.concat(
-                    PluginHandler.DEPRECATED_PLUGINS.stream(),
+                    PluginHandler.DEPRECATED_PLUGINS.stream().map(p -> p.name),
                     PluginHandler.UNMAINTAINED_PLUGINS.stream()
-            ).map(String::valueOf).collect(Collectors.joining(","));
+            ).collect(Collectors.joining(","));
             System.setProperty("josm.plugins", plugins);
             List<PluginInformation> list = PluginHandler.buildListOfPluginsToLoad(null, null);
Index: trunk/test/unit/org/openstreetmap/josm/tools/MultiMapTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/tools/MultiMapTest.java	(revision 15722)
+++ trunk/test/unit/org/openstreetmap/josm/tools/MultiMapTest.java	(revision 15723)
@@ -51,5 +51,5 @@
         map.remove("foo");
         assertEquals(null, map.get("foo"));
-        assertEquals("(alpha->{})", map.toString());
+        assertEquals("(alpha->[])", map.toString());
         map.remove("alpha");
         assertTrue(map.isEmpty());