Features: 1) Add suppress(NotRegistered) to handle unregistered translation models safely; 2) Enhance add_translations_fieldset logic for better error handling;

Fixes: 1) Add missing import for `NotRegistered` in `admin.py`;

Extra: 1) Minor refactoring of `add_translations_fieldset` function for readability;

core/admin.py

@@ -1,3 +1,5 @@
+from contextlib import suppress
+
 from constance.admin import Config
 from constance.admin import ConstanceAdmin as BaseConstanceAdmin
 from django.apps import apps
@@ -6,7 +8,7 @@ from django.contrib.admin import ModelAdmin, TabularInline
 from django.contrib.gis.admin import GISModelAdmin
 from django.db.models import Model
 from django.utils.translation import gettext_lazy as _
-from modeltranslation.translator import translator
+from modeltranslation.translator import NotRegistered, translator
 from modeltranslation.utils import get_translation_fields
 from mptt.admin import DraggableMPTTAdmin
 
@@ -43,16 +45,17 @@ class FieldsetsMixin:
     def get_fieldsets(self, request, obj=None):
         fieldsets = []
 
-        def add_translations_fieldset(fieldsets):
-            opts = translator.get_options_for_model(self.model)
+        def add_translations_fieldset(fss):
+            with suppress(NotRegistered):
+                transoptions = translator.get_options_for_model(self.model)
                 translation_fields = []
-            for orig in opts.local_fields:
+                for orig in transoptions.local_fields:
                     translation_fields += get_translation_fields(orig)
                 if translation_fields:
-                fieldsets = list(fieldsets) + [
+                    fss = list(fss) + [
                         (_("translations"), {"fields": translation_fields})
                     ]
-            return fieldsets
+            return fss
 
         if self.general_fields:
             fieldsets.append((_("general"), {"fields": self.general_fields}))