Warning: Undefined array key 2 in custom_field_tokens() (line 230 of modules/contrib/custom_field/custom_field.tokens.inc).

I am not sure my approach is the right one but it addresses the unexpected warning.

Can't currently produce this with RC12 version. If anyone else is having this issue, please help me reproduce.

This in theory should work as is and it is for me on multiple sites:
$property_name = $parts[2] ?? NULL;