Fixes exception raised when viewing repository settings

ee/app/models/ee/project_import_state.rb

@@ -97,6 +97,7 @@ module EE
 
     def mirror_update_due?
       return false unless project.mirror_with_content?
+      return false unless next_execution_timestamp?
       return false if project.archived?
       return false if hard_failed?
       return false if updating_mirror?

ee/changelogs/unreleased/212673-fix-undefined-method-exception.yml

+---
+title: Fix repository settings page loading issues for some imported projects with pull mirroring
+merge_request: 28879
+author:
+type: fixed

ee/spec/models/project_import_state_spec.rb

@@ -271,6 +271,15 @@ describe ProjectImportState, type: :model do
         expect(import_state.mirror_update_due?).to be false
       end
     end
+
+    context 'when next_execution_timestamp is nil' do
+      it 'returns false' do
+        import_state = create(:import_state, :finished, :mirror, :repository)
+        import_state.next_execution_timestamp = nil
+
+        expect(import_state.mirror_update_due?).to be false
+      end
+    end
   end
 
   describe '#last_update_status' do