I'm sure some people on both sides are thinking about it so the Secret Service is going to have to work over time. Personally if they haven't assassinated Obama, the first Black President of the United States, at this point (I hope it never happens; he has gotten this far) then the Secret Service has done a good enough job with security that I'm not too worried. Hillary Clinton is not nearly as threatening to Republicans as Obama ("the most liberal Democrat in the Senate who is a terrorist sympathizer and wasn't born in this country OMG!!!!"). She was a former first lady and if she wins will be the first female President. I think this is just talk by agitated Republicans who are worried that they are going to lose all power and brain-washed by Trump in to thinking the election is rigged.
I doubt it will happen. If it does happen there will not be a Civil War but there will probably be violence. At this point Trump has virtually no chance at winning so I'm not going to entertain the idea of a Trump Presidency any more. I will be relieved when CNN Projects Hillary Clinton as our next President but I don't sit around thinking about "What if he wins?" any more because his campaign has disintegrated.
So let the wingnuts be angry and talk about Civil War as much as they want. Most of them will get over it eventually.