Reproduction steps:
1. Create a new project
2. Create a new ShaderGraph file and add a few Properties in the Blackboard
3. Observe the Properties in the Blackboard window
4. Disable the Blackboard
5. Try to drag a Property from the last seen position

Expected result: No properties can be added since Blackboard is disabled
Actual result: Properties can still be dragged into the Graph

Reproducible with: 2019.2.0a1, 2019.2.7f2, 2019.3.0b5, 2020.1.0a7
Not reproducible with: 2017.4.32f1, 2018.4.10f1, 2019.1.14f1

Reproducible with package versions: 6.5.2, 6.9.1, 7.1.2
Not reproducible with package versions: 4.1.0, 5.16.1

Notes:
1. ShaderGraph 6.5.2 became available in 2019.2.0a1
2. The issue occurs only with new ShaderGraph files. If the project is re-opened, the already existing Graphs would not reproduce the issue.