Discovered in graph toolkit

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Steps to reproduce:

1. Install graph toolkit

## Open the package manager
## Click on the plus icon in the top-left corner and select "Install package by name"
## Insert "com.unity.graphtoolkit" in the name field
## Click on "Install"

1. Create and open a graph (Assets > Create > Graph Toolkit Samples > Simple Graph)
2. Press 'Space' to open the item library
3. Select any item and press 'Enter' to add it on the graph

Actual results: A null reference error is thrown

Expected results: No error is thrown

Reproducible with versions:
0.2.0-exp.1, 6000.3.0a4

Not reproducible with versions:
0.2.0-exp.1, 6000.3.0a1

Tested on (OS): MacOS