Reproduction steps:
1. Open the attached project "1334048.zip"
2. In the top toolbar open Window/Text/Font Asset Creator
3. Drag font "IBMPlexMono-Italic" into the Source Font of Font Settings
4. Click on the "Generate Font Atlas" button
5. Save the Font Asset
6. Try clicking around and opening windows like Package Manager or Animation window

Expected result: No error appears when opening Editor windows
Actual result: "NullReferenceException: Object reference not set to an instance of an object" error gets printed out in the Console every time you open some Editor window

Reproducible with: 2021.2.0a14, 2021.2.0a18
Not reproducible with: 2019.4.27f1, 2020.3.10f1, 2021.1.9f1, 2021.2.0a13
Couldn't test with: 2018.4 (Textcore package is not available)

Notes:
- Other types of errors appear on the creation of the Font Asset (attached in the edit)
- Creating Font Asset via right-clicking the Font and selecting Create/Text/Font Asset doesn't cause errors