Check if note's noteable is not nil when checking resolvability

This can occur when a note is added to a commit and then this commit
is deleted.

app/models/concerns/resolvable_note.rb

@@ -32,7 +32,7 @@ module ResolvableNote
 
   # Keep this method in sync with the `potentially_resolvable` scope
   def potentially_resolvable?
-    RESOLVABLE_TYPES.include?(self.class.name) && noteable.supports_resolvable_notes?
+    RESOLVABLE_TYPES.include?(self.class.name) && noteable&.supports_resolvable_notes?
   end
 
   # Keep this method in sync with the `resolvable` scope

changelogs/unreleased/jprovazn-fix-resolvable.yml

+---
+title: Fix resolvable check if note's commit could not be found.
+merge_request:
+author:
+type: fixed

spec/models/concerns/resolvable_note_spec.rb

@@ -326,4 +326,12 @@ describe Note, ResolvableNote do
       end
     end
   end
+
+  describe "#potentially_resolvable?" do
+    it 'returns false if noteable could not be found' do
+      allow(subject).to receive(:noteable).and_return(nil)
+
+      expect(subject.potentially_resolvable?).to be_falsey
+    end
+  end
 end