I would check all of the wiring, in sections. Get hold of a multimeter - screwfix do them cheap - and test the starter circuit for continuity. So the low power stuff, from the ignition switch to to the start button and to the relay. Then the high power stuff from the batter to the relay to the starter motor. Because of how a bikes wiring is routed, problems often occur around the steering yoke area and can be intermittent depending on the position of the bars, on full lock for example. Also check and clean the powered (+ve) part of the circuit. Check, and clean, all of the block and bullet connectors. Sounds like one of two things; a bad earth or something is earthing out or; a bad connection on the live side. Both commonly generate a lot of heat. Starter circuit shorting can cause fires.