This happens sometimes when I have a UIFlow tab open already for lets say an M5StickC Plus, then open another UIFlow browser tab for a 2nd device like the Core 2.
Because I selected Core 2 in the new browser tab, somehow the device selected in the other browser tab changes to Core 2 as well, and it shows that error the first time I run the program on the M5StickC until I update the device type again in that first tab.
I have no idea why the UIFlow tabs aren't fully isolated.