MariaDB [tpch1]> select calsettrace(1);
----------------

----------------

----------------
1 row in set (0.00 sec)

MariaDB [tpch1]> (select n_nationkey K from nation limit 1) union all (select n_nationkey K from nation limit 2) limit 1;
ERROR 2013 (HY000): Lost connection to MySQL server during query
MariaDB [tpch1]> select calsettrace(1);

In ha_calpont_impl_external_lock(), we try to access the connection info without checking if it exists (around lin 4406 of ha_calpont_impl.cpp):
else // vtable mode
{
if (thd->infinidb_vtable.vtable_state == THD::INFINIDB_SELECT_VTABLE)
{
if (ci->traceFlags & 1)
push_warning(thd, Sql_condition::WARN_LEVEL_NOTE, 9999, ci->cal_conn_hndl->queryStats.c_str());

ci->cal_conn_hndl is NULL in this case.