Problems upgrading project from 2018.2 to 2018.4.1 LTD

Hello!
Me and my team have been working on this project for about a year now and we decided that we needed some of the more recent Unity tools like ShaderGraph to start polishing our game so we decided to upgrade our project to the last version of 2018(fearing that upgrading to 2019 would give us problems). What happens is after upgrading most scripts can´t be loaded and some console errors that me as a newbie can´t really understand.

A visual example is all Text Mesh Pro texts gone and represented as a T image:


And this is what the console gives me:


Most solutions to the scripts require to make them from sratch but i wanted to avoid that as there is already alot of work on the game. I´ve never touched Porperty Drawers and there were no errors before the upgrade. Any help would be greatly appreciated!