Fwd: Segmentation fault on RelationGetDescr in my first extension

Hi!

I'm trying to write my first extension and open a table in it.
I use check_password_hook and my function executes at the moment of
changing user password.

But if I try to open a table inside this function, I get Segmentation fault
on the line with "RelationGetDescr".
What am I doing wrong?

void
_PG_init(void)
{
prev_check_password_hook = check_password_hook;
check_password_hook = check_password;
}

...

void check_password(const char *username, const char *shadow_pass,
PasswordType password_type, Datum validuntil_time, bool validuntil_null)
{
Relation rel;
TupleDesc tupdesc;
int Natts;

Oid tbl_oid = DatumGetObjectId(DirectFunctionCall1(to_regclass,
CStringGetTextDatum("my_test_table")));
if (OidIsValid(tbl_oid)) {
rel = table_open(tbl_oid, RowExclusiveLock);
tupdesc = RelationGetDescr(rel); // !!! server process (PID 70525) was
terminated by signal 11: Segmentation fault
Natts = tupdesc->natts;
table_close(rel, RowExclusiveLock);
} else {
ereport(NOTICE, (errmsg("IndexRelationId (%d) is NOT valid !", tbl_oid)));
}
}

Table is fine:

postgres=# \d my_test_table
Table "public.my_test_table"
Column | Type | Collation | Nullable | Default
--------+------+-----------+----------+---------
name | text | | |