Hopefully fixed errors with private fields

Author: JanHolger

src/main/java/org/javawebstack/abstractdata/mapper/MapperTypeSpec.java

@@ -98,7 +98,8 @@ public List<FieldSpec> getFieldSpecs() {
     }
 
     public Field getAdditionalField() {
-        additionalField.setAccessible(true);
+        if(additionalField != null)
+            additionalField.setAccessible(true);
         return additionalField;
     }