Pressing 'Enter' in the item library throws an error

Discovered in graph toolkit

—

Steps to reproduce:

1. Install graph toolkit
   1. Open the package manager
   2. Click on the plus icon in the top-left corner and select "Install package by name"
   3. Insert "com.unity.graphtoolkit" in the name field
   4. Click on "Install"
2. Create and open a graph (Assets > Create > Graph Toolkit Samples > Simple Graph)
3. Press 'Space' to open the item library
4. 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