[IMP] base: ensure correct inheritance chain in registry

When removing a custom model, it remained listed in the base classes
(normally `base` and also possibly a list of mixins; e.g. `mail.thread`
or `mail.activity.mixin`) list of inheriting classes, possibly causing a
crash when trying to reload the registry.

This commit ensures that any custom model is removed from its parent
class `_inherit_children` set; it will be re-added automatically during
the call to `_build_model` if the custom model still exists.

Co-Authored-By: Raphaël Collet <rco@odoo.com>

odoo/addons/base/models/ir_model.py

@@ -308,7 +308,7 @@ class IrModel(models.Model):
         # clean up registry first
         custom_models = [name for name, model_class in self.pool.items() if model_class._custom]
         for name in custom_models:
-            del self.pool.models[name]
+            del self.pool[name]
         # add manual models
         cr = self.env.cr
         cr.execute('SELECT * FROM ir_model WHERE state=%s', ['manual'])

odoo/modules/registry.py

@@ -183,6 +183,13 @@ class Registry(Mapping):
         """ Add or replace a model in the registry."""
         self.models[model_name] = model
 
+    def __delitem__(self, model_name):
+        """ Remove a (custom) model from the registry. """
+        del self.models[model_name]
+        # the custom model can inherit from mixins ('mail.thread', ...)
+        for Model in self.models.values():
+            Model._inherit_children.discard(model_name)
+
     def descendants(self, model_names, *kinds):
         """ Return the models corresponding to ``model_names`` and all those
         that inherit/inherits from them.