Yes, this is a bug. Lines 135 and 136 of MakGenUtility.js are:
if ( dirPath.indexOf( '/' ) != 0 )
throw new Error( "removeDirectory(): Relative directory." );
This is a safeguard test to prevent the uncontrolled removal of files outside a prescribed directory tree recursively, which could have disastrous consequences. The test works fine on UNIX/Linux filesystems. However, on Windows all absolute paths start with a "<u>:" drive specifier, so the root directory slash is not the first character of an absolute path on Windows.
I'll fix this problem ASAP with an update. Thanks for discovering it.