I am very happy that you want to continue your work on Unity and so you updated it recently.
You will be very happy to know that the error which you are facing is the known bug from Unity and I have the solution on this bug.
Please follow below steps to get rid of your errors.
Please Re-Import your Project from your Editor.
After importing it go to Assets -> Reimport All.
Then you need to go from MonoDevelop to following path Build->Clean All and then do the Build -> Rebuild All.
Finally you need to Close and restart your Unity and MonoDevelop.
Hope you will be able to run your updated Unity.