Several cases may arise:
You have access to the interface and you can go to Tools > Grids Editor to fix your code: correct it.
You get an error message as soon as you are in Store Commander's interface and you no longer have access to the Grids Editor. It is possible to disable the use of customized grids: insert "&setextension=0" in the Store Commander's URL and validate. Store Commander will restart without the customized grids and you can then fix your code.
Once your code is fixed, use '&setextension=1"' in the URL or go to Tools > Settings > Application > Use SC Extensions = 1.