For me on OSX it's under jdk1.8.0_05.jdk/Contents/Home/jre/lib/ext/jfxrt.jar. I'd guess it's in an analogous place on your machine. Try /usr/java/jdk1.8.0_05/jre/lib/ext/jfxrt.jar
Am using linuxmint 18.1 and i had the same issue. i realized that the open jdk i had didn't have the jfxrt.jar library.Download another jdk from oracle and then install it. Remember to change the path in the netbeans.conf file in the netbeans installation folder.