Config export fails with TypeError in SplitFilter

I encountered the same issue while updating from Config split 1.x to 2.x. The problem in my case was that during the upgrade process I had mixed up 1.x and 2.x configuration files for the splits when switching development branches.

Basically in my config_split.config_split.environment.yml file I still had the old blacklist and graylist keys instead of the keys that 2.x is using. Config export was expecting to find an array but because the key was missing, it gave the error.

The error was fixed by adding the correct keys to the config split configuration file:

complete_list: {  }
partial_list: { }