On Converting "Block Subgraph" to "Subgraph" from second time onwards, ArgumentNullException is thrown in the Console, refer attached text file for Stack Trace.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > Visual Effects Graph > Install
3. Project > Packages > Visual Effect Graph > Editor > Templates > Open Default Subgraph Block
4. Focus on "Block Subgraph"
5. Right-click on "Block Subgraph" and select "To Subgraph"
6. Again right-click on converted "Default Subgraph Block" and select "To Subgraph"

Actual Result:
ArgumentNullException is thrown on Converting "Block Subgraph" to "Subgraph" from second time onwards

Expected Result:
"Block Subgraph" convert to "Subgraph" multiple times without any exception thrown in the Console

Reproducible in:
2020.1.0a5, 2019.3.0b5, 2019.3.0a10

Package Version found: 7.1.2, 7.0.1, 7.0.0

NOTE:
Visual Effect Graph is introduced in 2019.3.0a10.

Environment:
Windows 10 & macOS 10.14