Run subgraph node does not detect the subgraph on blackboard asset

Steps to reproduce:

1. Create and open a blackboard asset
2. Add a subgraph variable in the blackboard asset
3. Create and open a behavior graph
4. Add the blackboard asset to the graph
5. Add a run subgraph node
6. Click on the chain icon on the node to open the pop-up for assigning variables

Actual results: The blackboard asset subgraph variable is not listed

Expected results: You can see the blackboard asset subgraph variable listed and you can select it and assign it

Reproducible with versions: 
1.0.9

Tested on (OS): MacOS