Hey -
Can you let me know if you have the same error occur in a new 4.12 project? If so, can you provide the setup steps so that I can try to reproduce this on my end? I setup my Level Editor - Play tab to match you screenshot but when I PIE I get an error message stating “The map specified on the commandline could not be found. Would you like to load the default map instead?” Can you let me know what other setup steps need to be done?