Fatal Error in gc : GetThreadContext Failed

Hi guys, i'm new with unty and those days i'm playing around with it.

I don't know it it's normal (ofcourse not) but everytime i use paint tool to define my terrain i get this thing out (see the pic). Sometimes at start, sometimes during the painting but everytime i use terrain tools it crash! I already uninstalled and re-installed Unity, i have placed it inside white list into my Avast antivirus, tried to close and re-open it and at least to restart my pc but nothing of those things worked. Thanks a lot.