As of Unreal 4.16.3-release the particle editor automation test asserts and fails due to the editor window being destroyed. To reproduce, run the Particle Editor automation tests in Window->DeveloperTools->Session Frontend. Under the automation tab, run System/Promotion/Editor/ParticleEditor. You may need to refresh tests for the test to show up.
The automated test is trying to send a keyboard command for saving the particle system to the slate application’s active top level window (ActiveTopLevelWindow), however at this point the window has already been destroyed and an assert is hit.