CREATE TABLE TABLE1 (
    ID   INTEGER NOT NULL
);

ALTER TABLE TABLE1 ADD CONSTRAINT PK_TABLE1 PRIMARY KEY (ID);

INSERT INTO TABLE1 (ID) VALUES (1);

ALTER TABLE TABLE1
ADD ID2 INTEGER
NOT NULL;

UPDATE TABLE1 SET ID2 = NULL;

The insert failed because a column definition includes validation constraints.
validation error for column "TABLE1"."ID2", value "*** null ***".

Invalid insert or update value(s): object columns are constrained - no 2 table rows can have duplicate column values.
violation of PRIMARY or UNIQUE KEY constraint "PK_TABLE1" on table "TABLE1".
Problematic key value is ("ID" = 1).

The update failed because a column definition includes validation constraints.
validation error for column "TABLE1"."ID2", value "*** null ***".
Key value of record is ("ID" = 1).

Invalid insert or update value(s): object columns are constrained - no 2 table rows can have duplicate column values.
violation of PRIMARY or UNIQUE KEY constraint "UNQ_INF$ABONENT$BT" on table "INF$ABONENT$BT".
Problematic key value is ("ID_ABONENT" = 555555, "ID_SERVICE" = 1, "ID_PERIOD" = 200912).

Invalid insert or update value(s): object columns are constrained - no 2 table rows can have duplicate column values.
violation of PRIMARY or UNIQUE KEY constraint "UNQ1_DIR$ABONENT$REASON_APPEAL" on table "DIR$ABONENT$REASON_APPEAL".
Problematic key value is ("NAME" = 'Квитанция').

ALTER TABLE TABLE1
ADD ID2 INTEGER
NOT NULL;