Overbeating is the mainly cause why frosting are too runny. If you add your dry ingredients just simply mix them until they combined and smooth. My solution for a too runny frosting is that I try chilling them for about 15-30 mins to firm it up and then I mix it.
![Winking face :wink: 😉](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png)