I am on Win 10 64bit. XLights crashes either while in the new Test or in the sequence tab after using the output. I reproduced this two ways.
1. Open the sequence then go to TEST turn on RGB output and then exit and go to sequence tab and then click output and then play.
2. Open the sequence, turn on output and then play.
I have two usb dongles configured and only one attached, so I am getting the usual popup that says the other is not attached. Possibly, this is related if no one else has seen this particular crash.
