Index: trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/FunctionsTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/FunctionsTest.java	(revision 18870)
+++ trunk/test/unit/org/openstreetmap/josm/gui/mappaint/mapcss/FunctionsTest.java	(revision 18875)
@@ -3,4 +3,5 @@
 
 import static org.junit.jupiter.api.Assertions.assertAll;
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
@@ -9,12 +10,19 @@
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
 import static org.openstreetmap.josm.data.osm.OsmPrimitiveType.NODE;
 
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
-
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
 import org.openstreetmap.josm.TestUtils;
 import org.openstreetmap.josm.data.coor.LatLon;
@@ -30,4 +38,5 @@
 import org.openstreetmap.josm.data.preferences.NamedColorProperty;
 import org.openstreetmap.josm.gui.mappaint.Environment;
+import org.openstreetmap.josm.gui.mappaint.MultiCascade;
 import org.openstreetmap.josm.gui.util.GuiHelper;
 import org.openstreetmap.josm.spi.preferences.Config;
@@ -35,4 +44,5 @@
 import org.openstreetmap.josm.testutils.annotations.MapPaintStyles;
 import org.openstreetmap.josm.testutils.annotations.Projection;
+import org.openstreetmap.josm.testutils.annotations.Territories;
 
 /**
@@ -62,4 +72,20 @@
             return new Environment(osm);
         }
+    }
+
+    private static Method[] FUNCTIONS;
+
+    static Stream<Method> getFunctions() {
+        if (FUNCTIONS == null) {
+            FUNCTIONS = Stream.of(Functions.class.getDeclaredMethods())
+                    .filter(m -> Modifier.isStatic(m.getModifiers()) && Modifier.isPublic(m.getModifiers()))
+                    .toArray(Method[]::new);
+        }
+        return Stream.of(FUNCTIONS);
+    }
+
+    @AfterAll
+    static void tearDown() {
+        FUNCTIONS = null;
     }
 
@@ -231,3 +257,34 @@
         assertTrue(Functions.parent_osm_primitives(env, "type2").isEmpty());
     }
+
+    /**
+     * Non-regression test for #23238: NPE when env.osm is null
+     */
+    @ParameterizedTest
+    @MethodSource("getFunctions")
+    @Territories // needed for inside, outside, is_right_hand_traffic
+    void testNonRegression23238(Method function) {
+        if (function.getParameterCount() >= 1 && function.getParameterTypes()[0].isAssignableFrom(Environment.class)
+         && !function.getParameterTypes()[0].equals(Object.class)) {
+            Environment nullOsmEnvironment = new Environment();
+            nullOsmEnvironment.mc = new MultiCascade();
+            Object[] args = new Object[function.getParameterCount()];
+            args[0] = nullOsmEnvironment;
+            for (int i = 1; i < function.getParameterCount(); i++) {
+                final Class<?> type = function.getParameterTypes()[i];
+                if (String.class.isAssignableFrom(type)) {
+                    args[i] = "";
+                } else if (String[].class.isAssignableFrom(type)) {
+                    args[i] = new String[] {"{0}", ""}; // join and tr require at least 2 arguments
+                } else if (Double.class.isAssignableFrom(type) || double.class.isAssignableFrom(type)) {
+                    args[i] = 0d;
+                } else if (Object.class.isAssignableFrom(type)) {
+                    args[i] = new Object[0];
+                } else {
+                    fail(type.getCanonicalName());
+                }
+            }
+            assertDoesNotThrow(() -> function.invoke(null, args));
+        }
+    }
 }