Avoid exception when validating diff_note support

app/models/diff_note.rb

@@ -149,7 +149,7 @@ class DiffNote < Note
   end
 
   def supported?
-    for_commit? || for_design? || self.noteable.has_complete_diff_refs?
+    for_commit? || for_design? || self.noteable&.has_complete_diff_refs?
   end
 
   def set_line_code

changelogs/unreleased/kassio-diff-note-avoid-exception-when-validating-style.yml

+---
+title: Avoid exception when validating diff_note support
+merge_request: 48187
+author:
+type: fixed

spec/models/diff_note_spec.rb

@@ -38,6 +38,14 @@ RSpec.describe DiffNote do
     it_behaves_like 'a valid diff positionable note' do
       subject { build(:diff_note_on_commit, project: project, commit_id: commit_id, position: position) }
     end
+
+    it "is not valid when noteable is empty" do
+      note = build(:diff_note_on_merge_request, project: project, noteable: nil)
+
+      note.valid?
+
+      expect(note.errors[:noteable]).to include("doesn't support new-style diff notes")
+    end
   end
 
   describe "#position=" do