How to reproduce:
1. Open the attached "TerrainTools PManager Issue.zip" project
2. Open the Package Manager
3. Wait a couple of seconds until "Loading packages..." is done
4. Change filter from "In Project" to "All Packages" by using the Package Manager's drop-down menu
5. Observe the "NullReferenceException" error in the Console
6. Click on some of the Package names and observe the right side of the Package Manager

Expected result: filtered packages are shown and selected package information is reflected on the right side
Actual result: Package Manager filter does not work and selected package information is not reflected on the right side

Reproduced in: 2019.2.5f1 (TerrainTools 1.1.4 version)
Not reproduced in: 2019.2.5f1 (TerrainTools 2.0.0 version), 2019.3.0b3 (TerrainTools 2.0.0 version), 2020.1.0a4 (TerrainTools 2.0.0 version)
Can not reproduce in: 2017.4.32f1 (no TerrainTools package), 2018.4.8f1 (no TerrainTools package), 2019.3.0b3 and 2020.1.0a4 (with TerrainTools 1.1.4 installed "Package manager extension failed" error appears, therefore it was not possible to reproduce same issue)

Notes:
It only happens with Terrain Tools package versions 1.1.2 and 1.1.4.
Upgrading the Terrain Tools package to version 2.0.0 and above fixes the issue.

Workaround: switch Package Manager filters before "Loading packages..." is finished. This will still cause Console error, but both filtering and information reflecting will work.