Well, now that we know that it's a 250 Ninja we can get somewhere.
The switch is definitely the sidestand switch. I attached a photo of mine to show you what it looks like when it's properly installed. Let me know if you need a better picture. When the switch is released (like it is on your bike right now) the engine can run. When you press in the button (by putting down the side stand) the switch kills your engine if you're in gear and the clutch is released. The bike will run fine as is, but you may want to fix it anyway, especially if it just needs to get screwed back on.
These switches fail sometimes, and when they do some people bypass the switch rather than fix it. That may be what the previous owner did with this one.
As for the tube, I'm not sure. That will take a little more looking.
Curt