Asset store being glitchy

I have no idea what is happening but, every time I import an asset from the assets store, Something breaks and makes my game unfixable,

Example: I added the 3D Game Kit (https://assetstore.unity.com/packages/templates/tutorials/3d-game-kit-115747) and all of the collision in my game would break, resulting in everything going through everything.

Example 2: Just now, I got War FX (https://assetstore.unity.com/packages/vfx/particles/war-fx-5669) and my FPS gun went permanently invisible.

Can someone provide a solution? Because every time this happens I have to delete the project and restart.

 

And this is why backups of your entire Project are very important, if not mandatory.
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Unfixable is a... strong word.

 

Use source control for your project (GIT, SVN etc). This will remove the fear of breaking your project and allow you to freely tinker without consequence.

If you are using source control, you can perform a diff on the project before and after adding the asset in question and see what has changed. For the first example, I suspect that adding the 3D Game Kit asset may have somehow messed with the physics settings (especially layers).

EDIT: I just noticed that the 3D Game Kit contains a "Project Settings" folder. This is probably overriding your own project settings and breaking your project. Try importing the 3D Game Kit minus the Project Settings folder. This may fix your problem.