A coding error caused the tree node indicators to be kept at the top of the list instead of being removed from it when the child processes were being loaded. When sorting the list, these indicators failed to sort and the exception was displayed.
This issue has been solved and the nodes are now correctly removed when the child processes finish loading.
Please update to a Fix Version
A fix will be available with the following versions:
Automic Engine Version 12.2.0 - available to download
Automic Engine Version 12.0.6 - planned release September, 12 2018