Support for string attribute on fields in pivot view was added in 24f7c7ce7f
In some cases, the field is not stored and read_group method is
overridden to allow its use in a pivot view. Such a field does not
appear in the measures dict trying to set the string attribute, leading
to a client crash.
With this commit, the string attribute is set after the non stored
fields were added to the dict.
closes odoo/odoo#29393