I just updated my ElementaryOS from Juno to Hera. When I open System Settings and try to open Tweaks. This closes the entire System Settings window and does not open the Tweaks window.
You have to update elementary-tweaks package also in order to work with Hera. When the update to Hera was released, elementary tweaks behave like you explain, but after a while it was updated also and now it works as before, only slingshot options for raws and columns are missing, because it was removed from the OS, only 5x3 grid is presented at the moment. Execute
sudo apt update && sudo apt upgrade into terminal.
Another possibillity is that you have set something before the update which is not supported now. I can advice you to remove all you settings made by elementary-tweaks by purging and reinstalling it. In order to do that, execute
sudo apt purge elemetnary-tweaks
if you do not have purge installed, you can do this by
sudo apt install purge. After purging install it again:
sudo apt install elementary-tweaks