Null Pointer Exception while mocking Optional Parameters

My code is of the form:

public String getDeliveryDestinations(final RequestContext requestContext {
    final Accessor accessor = requestContext.getAccessor().get();

    final GroupAccessor groupAccessor = accessor.getGroupAccessor();

    final String groupId = groupAccessor.getGroupId();


    if(groupId.equals("s1") return "Type1";
    else return "type 2";


}

For testing this, i wrote the following:

@Test
public void getDestinationsDestinations_empty() throws Exception {


    Accessor accessor = mock(Accessor.class);
    GroupAccessor groupAccessor = mock(GroupAccessor.class);
    //*mockRequestContent is Mocked*
    when(mockRequestContext.getAccessor().get()).thenReturn(accessor);
    when(accessor.getGroupAccessor()).thenReturn(groupAccessor);
    when(groupAccessor.getGroupId()).thenReturn("s1");

    String destinationResult = provider.getDeliveryDestinations(mockRequestContext);
    assertEquals(destinationResult,"Type1");

}

I get a NPE at, when(mockRequestContext.getAccessor().get()).thenReturn(accessor); I don't know why!

Any leads would be very helpful :)