Avoid 500's when commit is not reachable

app/models/diff_note.rb

@@ -77,7 +77,7 @@ class DiffNote < Note
   end
 
   def supports_suggestion?
-    return false unless noteable.supports_suggestion? && on_text?
+    return false unless noteable&.supports_suggestion? && on_text?
     # We don't want to trigger side-effects of `diff_file` call.
     return false unless file = latest_diff_file
     return false unless line = file.line_for_position(self.position)

changelogs/unreleased/osw-avoid-500-on-suggestions-check.yml

@@ -0,0 +1,5 @@
+---
+title: Avoid 500 when rendering users ATOM data
+merge_request: 25408
+author:
+type: fixed

spec/models/diff_note_spec.rb

@@ -321,6 +321,14 @@ describe DiffNote do
   end
 
   describe '#supports_suggestion?' do
+    context 'when noteable does not exist' do
+      it 'returns false' do
+        allow(subject).to receive(:noteable) { nil }
+
+        expect(subject.supports_suggestion?).to be(false)
+      end
+    end
+
     context 'when noteable does not support suggestions' do
       it 'returns false' do
         allow(subject.noteable).to receive(:supports_suggestion?) { false }