Just imported Shapes to Unity 2019.4 - getting tons of packate-related compiler errors (similar to existing bug, but existing solution not available) -- but! solution found, documentation update needed.

I just downloaded and imported Shapes 3.2.3 into Unity 2019.4.19f1 and am unable to run any of the scenes under Samples. It's throwing a bunch of errors like: "Library\PackageCache\com.unity.package-manager-ui@2.0.13\Editor\AssemblyInfo.cs(7,12): error CS0246: The type or namespace name 'UxmlNamespacePrefixAttribute' could not be found (are you missing a using directive or an assembly reference?)"

I see that this may be the same as a bug that's been reported before (https://shapes.userecho.com/en/communities/1/topics/38-installation-error-unity-20193), but the solution suggested for that one doesn't seem to apply: that solution was to delete the "Package Manager UI package." However, I don't see this in the Package Manager, and from the Unity online docs (https://docs.unity3d.com/Manual/upm-errors.html) it shouldn't be there any longer: 
"As of 2019.3, your manifest.json file should not contain any references to the com.unity.package-manager-ui package." 

Just to be certain, I went to the manifest file and found this line:

"com.unity.package-manager-ui": "2.0.13",

Which I then deleted (per the Unity documentation cited above). However it made no difference: I restarted Unity, re-imported Shapes, and found the same errors.

HOWEVER: I then went back one more time, deleting that line from the manifest file once again, and this time when the popup saying "This Unity Package has Package Manager dependencies", I chose "Skip" instead of  "Install/Upgrade"

... and now it appears to work. 

So really, I think what's probably needed here is an update to the Shapes documentation, especially the Quick Start guide, so people don't get stopped by this upon install.