This is a common issue with TGMPA, I have seen it before.
Some themes do not load TGMPA correctly because they assume that no other plugin is using that same library.
Simply delete the file
This should resolve the problem.
Ideally you should notify the theme developer about this. All themes and plugins must check if TGMPA is defined before attempting to redefine it. Unfortunately some themes do not include the latest version of TGMPA that does this extra check.