Steps to reproduce:
1. Open Window->UI Toolkit->UI Builder
2. Select the Project tab in the Library menu
3. Open the UnityEngine->UIElements dropdown item
4. Scroll down to find an item called "Instance"
5. Hover over it to see an error in the console

Expected result: No errors and perhaps no Instance item in the Library?
Actual result: A seemingly pointless Instance item is available to the user and an error is thrown.

Reproducible with: 2021.1.20f1, 2021.2.8f1, 2022.1.0a1, 2022.1.0b2, 2022.2.0a9,

Could not resolve template with name ''
UnityEngine.UIElements.UxmlFactory`2<UnityEngine.UIElements.TemplateContainer, UnityEngine.UIElements.TemplateContainer/UxmlTraits>:Create (UnityEngine.UIElements.IUxmlAttributes,UnityEngine.UIElements.CreationContext)
Unity.UI.Builder.BuilderLibraryProjectScanner/<>c__DisplayClass5_1:<ImportFactoriesFromSource>b__0 ()
Unity.UI.Builder.BuilderLibraryView:OnItemMouseEnter (UnityEngine.UIElements.MouseEnterEvent)
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)