"Value cannot be null"

Hello all,
Sorry to be a newbie; I am a couple years out of practice. When I try to compile and run my game I get the error message:

Value cannot be null. (Parameter 'value)

Any idea on what I'm missing? Thanks for any help

Here is the actual error with a couple lines of context, in case that would help:

"TextureGroups":[...

"Value cannot be null"