To reproduce:

1. Go to Window -> Package Manager
2. Select Package Manager UI
3. Press Update to 1.6.1
4. Observe the console

Expected: no errors show up in the console

Reproduced in 2018.1.0b5
Could not test in the earlier versions, there is no Package Manager window.
Could not the in the later versions, the Package Manager UI is already updated to 1.6.1

Won't fix: starting with Package Manager UI 1.6.1 only, there s now a popup dialog that asks you to confirm update of Package Manager UI because it requires closing the Package Manager UI window temporary (code recompilation).
So if you update Package Manager UI prior to 1.6.1, NullReference errors will appear in the console. The workaround is to close the project and reopen it.