Error in COT Code Generation

I have an application which has been working nicely. However, I started having an error in the COT generator not being to edit any controller. This error pops up as soon as I right click to make a change.

At the moment am unable to edit or make changes to any of the controllers.
I have tried to restore the application from a backup, but it seems to behave the same manner.
I have removed any custom CSS that was being used.

Could someone point me in the direction how to get rid or resolve this?
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