Fix edit not available in in-app messages Admin section

kobo/apps/help/admin.py

@@ -16,17 +16,12 @@ class InAppMessageAdmin(MarkdownxModelAdminBase):
     )
     readonly_fields = ['uid', 'last_editor']
 
-    def get_form(self, *args, **kwargs):
+    def get_fieldsets(self, request, obj=None):
         """
         Allows us to add warning messages to the top of the form without
         manually maintaining a list of model fields in this class. An
         approximation of a model-level `help_text`
         """
-
-        # Get the auto-generated form, which will contain the appropriate list
-        # of fields, from the superclass
-        form = super().get_form(*args, **kwargs)
-
         # Next, build `fieldsets` using our warning messages and that list of
         # fields. From the Django documentation:
         #     fieldsets is a list of two-tuples…in the format (name,
@@ -35,13 +30,15 @@ def get_form(self, *args, **kwargs):
         #     about the fieldset, including a list of fields to be displayed in
         #     it.
         #     The name is effectively a heading. To use multiple headings as
-        #     for warnings, a second field set must be set with with the list
+        #     for warnings, a second field set must be set with the list
         #     of fields set to an empty string.
-        # https://docs.djangoproject.com/en/2.2/ref/contrib/admin/#django.contrib.admin.ModelAdmin.fieldsets
-        self.fieldsets = [
-            (self.new_message_warning, {'fields': ''}),
-        ]
-        return form
+        # https://docs.djangoproject.com/en/3.2/ref/contrib/admin/#django.contrib.admin.ModelAdmin.fieldsets
+
+        fieldsets = super().get_fieldsets(request, obj)
+        # Only display messages on edit.
+        if obj:
+            fieldsets.insert(0, (self.new_message_warning, {'fields': ''}))
+        return fieldsets
 
     def save_model(self, request, obj, form, change):
         obj.last_editor = request.user