Ok, small follow-up.
I’ve managed to fix this problem using the info in this thread: [Question] Null values causing errors in Perspective
I just added a validation Script (it can check for “None”), Expressions worked, but it still gave off a warning, albeit not crashing the gateway.
The problem was caused by the lack of selection in the Tree. Since the “selection” and “selectionData” properties on the Tree are dynamic (and therefore empty if nothing is selected) it caused problems on the binding.
These problems/warnings on the binding are to be expected, but they shouldn’t crash the gateway, so, maybe a bug?