Testcase: Note that the error 1048 query (INSERT INTO t VALUES (NULL);) is not required for reproduction of the 126 error, it is there only to give a clue about what the underlying issue may be.

SET sql_mode='';

CREATE TABLE t (c INT KEY) ENGINE=Aria;

INSERT INTO t VALUES (1);

REPAIR TABLE t QUICK USE_FRM;

--error 1048   # ERROR 1048 (23000): Column 'c' cannot be null

INSERT INTO t VALUES (NULL);

--error 126    # ERROR 126 (HY000): Got error '126 "Index is corrupted"' for './test/t.MAI'

UPDATE t SET c=NULL;

Leads to:

11.0.1-dbg>UPDATE t SET c=NULL;                        

ERROR 126 (HY000): Got error '126 "Index is corrupted"' for './test/t.MAI'

Bug confirmed present in:
MariaDB: 10.6.12 (dbg), 10.6.12 (opt), 10.7.8 (dbg), 10.7.8 (opt), 10.8.7 (dbg), 10.8.7 (opt), 10.9.5 (dbg), 10.9.5 (opt), 10.10.3 (dbg), 10.10.3 (opt), 10.11.2 (dbg), 10.11.2 (opt), 11.0.1 (dbg), 11.0.1 (opt)

Bug (or feature/syntax) confirmed not present in:
MariaDB: 10.3.38 (dbg), 10.3.38 (opt), 10.4.28 (dbg), 10.4.28 (opt), 10.5.19 (dbg), 10.5.19 (opt)