I can't start the MCreator client test environment, maybe this was patched already in the latest update but I can't upgrade or else I lose all my stuff from this version. (I don't even know if the issue is platform dependent or not, but something tells me "This mapping 'snapshot_20171003' was designed for MC 1.12! Use at your own peril." is the main reason)
Since I don't know exactly what information you need, I added a .zip file containing the gradle log file.
Edit: I also get the same error in the latest version, so I'm updating the MCreator version.