Strange. What's your system setup (e.g. your OS, version of R, version of RStudio — see how to get that info in post below)?
Just so we can be sure we're running the same code, let's use this example (see reprex, below). Definitely start a fresh session to make sure you don't have anything funky going on in your environment that's somehow colliding.
library(ggplot2)
ggplot(mtcars) +
geom_point(aes(mpg, hp))

Created on 2019-01-31 by the reprex package (v0.2.1.9000)