Upgrade 6->7 problem. node_gallery_api_menu can trigger PDOException in menu_router rebuild and cripple the site

I'm just documenting this here, as it took me several hours to trace. But (I hope) it's an edge case that doesn't actually need work. Though there is a place it can be improved.

Symptom:
at the far end of a migration, after doing as much of the content conversions (entityreference fields etc) on my huge site, and eventually finally doing the node_gallery update.php, things were working.
Until I cleared the cache.
At which point, 2/3 of my admin menu items went away.

After a few DB restores and all sorts of random poking around, I found that
* clearing the cache - rebuilding the menu router - crashed part-way through, which is why most of my site didn't work.

Further tracing and debug narrowed it down to an error (seen through drush cc menu) that started like:

WD menu: PDOException: SQLSTATE[23000]: Integrity constraint         ESC[31;40mESC[1m[error]ESC[0m
violation: 1048 Column 'title' cannot be null: INSERT INTO
{menu_router} ......

Additional debug narrowed the offending insert to one generated by node_gallery_api_menu()

WD menu: PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'link_title'      [error]
cannot be null: INSERT INTO {menu_links} (menu_name, plid, link_path, hidden, external, has_children,
expanded, weight, module, link_title, options, customized, updated) VALUES (:db_insert_placeholder_0,
:db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3,
:db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6,
:db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9,
:db_insert_placeholder_10, :db_insert_placeholder_11, :db_insert_placeholder_12); Array
(
    [:db_insert_placeholder_0] => navigation
    [:db_insert_placeholder_1] => 66971
    [:db_insert_placeholder_2] => node/%/upload/node_gallery_image
    [:db_insert_placeholder_3] => -1
    [:db_insert_placeholder_4] => 0
    [:db_insert_placeholder_5] => 0
    [:db_insert_placeholder_6] => 0
    [:db_insert_placeholder_7] => -2
    [:db_insert_placeholder_8] => system
    [:db_insert_placeholder_9] => 
    [:db_insert_placeholder_10] => a:0:{}
    [:db_insert_placeholder_11] => 0
    [:db_insert_placeholder_12] => 0
)
 in menu_link_save() (line 3126 of .../includes/menu.inc).

To skip to the end ...

* Our site (for reasons lost in time) once had TWO node_gallery relationships, as seen at /admin/config/content/node-gallery.
* One of those relationships, now unused, referred to a content type (node_gallery_image) that had since been deleted from the system (we'd replaced it with a generic 'image' content type that was used more widely)
* When the content type was deleted, the nodegallery relationship was invalidated, but not removed
And now, years later, the code found in D7 hook_menu :
- lists the relationships, including the broken one
- tries to load a link to that content type and get its name
- does not validate it's OK
- then tries to make a menu item named after it
- core menu does not validate it either
= and the database dies.

Now I've found it, I just want to cry, but OTOH, I think this problem is so far out there nobody else should ever hit it.
But to get my site online again, I went in to node_gallery_api.module:163~
and put in a check and an early exit.

      foreach ($relationship->item_types as $item_type) {
        $type_info = node_type_load($item_type);
        if (! $type_info) {
          watchdog(__FUNCTION__, "Invalid type referred to - can't init %item_type. You probably need to review the content types mentioned in the  node_gallery relationships.", array('%item_type' => $item_type), WATCHDOG_ERROR);
          continue;
        }
        $items['node/%node_gallery_api_gallery/upload/' . $item_type] = array(

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