Suddenly there is so many errors when I open a new Unity project,

I've started learning Unity recently, and everything was working fine, then I uninstalled Kaspersky internet security, and then I faced up some problems in Windows but I solved it.
Then today I wanted to open an old project in Unity that worked fine days ago, and I have a lot of errors like this Error: Could not load signature of Packages.Rider.Editor.UnitTesting.TestEvent:.ctor due to: Could not load file or assembly 'nunit.framework, Version=3.5.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb' or one of its dependencies. assembly:nunit.framework, Version=3.5.0.0, Culture=neutral, PublicKeyToken=2638cd05610744eb type: member:(null) signature: about 450 errors like that :) ![unity errors][1] I tried to uninstall the editor and reinstall it, BUT nothing works.
I also tried to start a new 3D project, and it also ended with the same result.
the unity editor I used is 2019.4.18f1 Personal
Need some help pls :( [1]: /storage/temp/174804-screenshot-2021-01-24-233922.png ,