This is a known bug - if you close Runner or if you have your preferences set up so that only the Builder or Coder window opens on launch, then the app’s internal representation of Runner is just None and so any attempt to send things to the output window causes this error. To fix, just set your preferences to open Builder, Coder and Runner on launch and try to avoid closing the Runner.
2020.2.5 will be out soon and will have this issue fixed!