Random Popup Window from App Store. Help?

Back up all data before proceeding.

First, if you installed the "San Francisco" font, remove it and restart. Otherwise, see below.

Launch the Font Book application and validate all fonts. You must select the fonts in order to validate them. See the built-in help and this support article for instructions. If Font Book finds any issues, resolve them.

Start up in safe mode to rebuild the font caches. Restart as usual and test.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t start in safe mode. In that case, ask for instructions.

If you still have problems, then from the Font Book menu bar, select

File ▹ Restore Standard Fonts...

You'll be prompted to confirm, and then to enter your administrator login password.

Also note that if you deactivate or remove any built-in fonts, for instance by using a third-party font manager, the system may become unstable.

This should fix it for you:

Fix Font Corruption

Open Terminal in your Utilities folder and paste the following after the prompt:

fontrestore default

Press RETURN.

If this doesn’t fix the problem then paste the following after the prompt:

sudo atsutil databases -remove

Press RETURN.

You may need to restart the computer for anything to take effect.

Booting into Safe Mode isn't required nor is Font Book nor do you need any articles downloaded. Contrary to the belief of some users lengthy explanations are not necessarily any better than short ones. Those users want to suggest that a long answer is correct while a short one isn't.

You should be aware that running the fontrestore command in a shell has the same effect as Restore Standard Fonts in Font Book; namely, all fonts that you installed will be deactivated. That destructive action is almost certainly not needed.