Is that when you start a new game in playtest or when you continue a saved game?
Whenever you add or remove a plugin, there is a very high chance that older savefiles become incompatible because most plugins need to insert their data into the savefiles.
And that is one of the many errors that can happen as the result of loading an incompatible savefile.
The other possibility is that you didn't configure a plugin correctly, or that it is incompatible to one of the other plugins you already installed.
In those cases we need to know what plugin you tried to install (link to the website where you got it), the full error trace of that error (press F8 when it comes up next time and give a screenshot of the console instead of this screen) and a screenshot of your plugin manager with all the plugins you have installed and their order.