message: Only update render appearance settings if there's a renderer
  I ran into this crash when I just opened a new tab.
  Fixes
diff: @@ -481,12 +481,15 @@ namespace winrt::Microsoft::Terminal::Control::implementation
  // Update the terminal core with its new Core settings
  _terminal->UpdateAppearance(newAppearance);
  + if (_renderEngine)
  + {
  // Update DxEngine settings under the lock
  _renderEngine->SetSelectionBackground(til::color{ newAppearance.SelectionBackground() });
  _renderEngine->SetRetroTerminalEffect(newAppearance.RetroTerminalEffect());
  _renderEngine->SetPixelShaderPath(newAppearance.PixelShaderPath());
  _renderer->TriggerRedrawAll();
  }
  + }
  // Method Description:
  // - Style our UI elements based on the values in our _settings, and set up
  