Not everything that claims to help stop malicious content, is actually free of malicious content itself

In this case, it's likely Windows Defender is doing its job properly and it wasn't a false positive, but you can check its log to be sure. To do so, load Windows Defender and click the History tab.
Attachment:
imgWindowsDefender-ViewThreats.png [ 72.56 KiB | Viewed 2898 times ]
/edit
I've checked Unchecky myself, and am not seeing anything malicious, so if WD was detecting Unchecky itself, it is an F/P. The Windows Defender logs will allow determination as far as the specifics go.