Update hook 7106, in specific this line https://github.com/Islandora/islandora_xml_forms/blob/7.x/builder/xml_form_builder.install#L525 fails if there is an actual XML association with a NULL value for XSLT transform value.
This value can be set as NULL if MODS to DC is enabled as a global derivative option, which basically removes all those XLST Form fields from the admin association UI when selected.
A possible options to fix this would be
A) Even when MODS to DC is selected as a derivative, if we require that field to be not null, maybe we could force that field to be a default XSLT (even when invisible)
B) Change 7106 update hook so it does not force that Table field to be null and create a new 7107 hook that fixes existing values and then sets, in the schema, it to be NULL again.
Tagging Jonathan Green