Com.unity.package-manager-ui errors in log on upgrading project to 2018.1.0b5 to b6.
There is no ability to run the project with this errors in log. The workaround is to reopen project: in this case, errors are gone.

To repro:
1. Open project attached in 2018.1.0b5.
2. Save it and open it in 2018.1.0b6.

Actual result:
On opening excisting project in 2018.1.0b5 or upgrading it to 2018.1.0b6 errors fall in log:

/Users/asyasmirnova/Library/Unity/cache/packages/packages.unity.com/com.unity.package-manager-ui@1.5.1/Tests/Editor/Services/Packages/PackageInfoTests.cs(1,7): error CS0246: The type or namespace name `NUnit' could not be found. Are you missing an assembly reference?
/Users/asyasmirnova/Library/Unity/cache/packages/packages.unity.com/com.unity.package-manager-ui@1.5.1/Tests/Editor/Services/Packages/PackageTests.cs(4,7): error CS0246: The type or namespace name `NUnit' could not be found. Are you missing an assembly reference?

Expected result: No errors in the log.