Or you could re-write the view to avoid the problem:

CREATE VIEW TODOS2

AS

SELECT A.rgt, A.codctt, D.CIF, D.morada, D.dta_alteracao, D.bictb

FROM allt AS A

JOIN

(

SELECT B1.codctb, B1.CIF, B1.morada, B1.dta_alteracao, B1.bictb

FROM contribuintesest AS B1

UNION ALL

SELECT B2.codctb, B2.CIF, B2.morada, B2.dta_alteracao, NULL

FROM contribuintee AS B2

UNION ALL

SELECT B3.codctb, B3.CIF, B3.morada, B3.dta_alteracao, NULL

FROM contribuinteinst B3

) D

ON A.codctb = D.codctb