I’ve sent a pull request adding a filter which allow hiding this warnings:
https://github.com/duracelltomi/gtm4wp/pull/243
Same problem here. Isn’t just the case of set $cwa_widget_wrappers as an empty array when json decode return an empty string? I didn’t make a deep review of the code but in my case I saw that $cwa_widget_wrappers is an empty string trying to be iterated in a foreach.