Fix #4878: NPE for Map-converting serializer

src/test/java/com/fasterxml/jackson/databind/convert/MapConversion4878Test.java

@@ -0,0 +1,59 @@
+package com.fasterxml.jackson.databind.convert;
+
+import java.util.Collections;
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import com.fasterxml.jackson.databind.module.SimpleSerializers;
+import com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer;
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+import com.fasterxml.jackson.databind.util.StdConverter;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class MapConversion4878Test extends DatabindTestUtil
+{
+    // [databind#4878]
+    static class MapWrapper4878 {
+        final Map<String, Object> value;
+
+        MapWrapper4878(Map<String, Object> value) {
+            this.value = value;
+        }
+    }
+
+    static class WrapperConverter4878 extends StdConverter<MapWrapper4878, Object> {
+        @Override
+        public Object convert(MapWrapper4878 value) {
+            return value.value;
+        }
+    }
+
+    @SuppressWarnings("serial")
+    static class Serializers4878 extends SimpleSerializers {
+        @Override
+        public JsonSerializer<?> findSerializer(SerializationConfig config, JavaType type, BeanDescription beanDesc) {
+            Class<?> rawClass = type.getRawClass();
+            if (MapWrapper4878.class.isAssignableFrom(rawClass)) {
+                return new StdDelegatingSerializer(new WrapperConverter4878());
+            }
+            return super.findSerializer(config, type, beanDesc);
+        }
+    }
+
+    // [databind#4878]
+    @Test
+    public void testMapConverter() throws Exception
+    {
+        SimpleModule sm = new SimpleModule();
+        sm.setSerializers(new Serializers4878());
+        final ObjectMapper mapper = jsonMapperBuilder()
+                .addModule(sm)
+                .build();
+        String json = mapper.writeValueAsString(new MapWrapper4878(Collections.singletonMap("a", 1)));
+        assertEquals(a2q("{'a':1}"), json);
+    }
+}