assertNull(concreteImplementation.getLink());
            });
        }

        @Test
        @DisplayName("Should handle null key in setKey")
        void testSetKeyWithNull() {
            // Test with mock
            doNothing().when(mockReferralData).setKey(null);
            assertDoesNotThrow(() -> mockReferralData.setKey(null));

            // Test with concrete implementation