NullPointerException when Spring SecurityContextHolder is called

I have been trying to write a test case for the following line of code but I keep getting java.lang.NullPointerException, I have tried to follow/replicate what others have suggested here Unit testing with Spring Security but I have had no luck. Can someone please help me better identify or give me a hint what I need to do. (I'm using mockito for this)

Code:

if (SecurityContextHolder.getContext().getAuthentication().getPrincipal().equals(user)) {
                continue;
            }

Test case:

@Test
public void testExpireAllSession() throws Exception {

        SecurityContext securityContext = Mockito.mock(SecurityContext.class);
        Mockito.when(securityContext.getAuthentication().getPrincipal().equals(any(Object.class))).thenReturn(false);
        SecurityContextHolder.setContext(securityContext);

       controller.theMEthodUnderTest();
}

..