What Java version do you use (see in About dialog) ?

I think it's a problem in Java. If you installed Sweet Home 3D with Java Web Start try to update the Java version installed on your system. If you installed Sweet Home 3D with the installer, try to update the Java version installed on your system and then use the Java Web Start version of Sweet Home 3D available at http://sweethome3d.sourceforge.net/download.html

See also: https://sourceforge.net/tracker/index.php?func=detail&aid=1860446&group_id=152568&atid=784665