Bug Using a template from a custom package sometimes throws errors

Because we have multiple Unity projects that we want to use the same GUI style for I created a package which contains our default controls, such as buttons, with their default styling. This seems to work fine in the UI Builder, I can add them to my GUI and they show up in runtime.

Occasionally though when I hit save in the UI Builder I get a set of errors "Template declaration requires a path or src attribute referencing another UXML file" and then "Unknown template name Button". Full errors below:

Spoiler: full errors

Code (CSharp):

1. Assets/Scripts/AnimationSelectionTree.uxml (1,2): Semantic - 'Template' declaration requires a 'path' or 'src' attribute referencing another UXML file
2. UnityEditor.AssetDatabase:Refresh ()
3. Unity.UI.Builder.BuilderDocumentOpenUXML:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Document/BuilderDocumentOpenUXML.cs:475)
4. Unity.UI.Builder.BuilderDocument:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Document/BuilderDocument.cs:271)
5. Unity.UI.Builder.BuilderToolbar:SaveDocument (bool) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Toolbar/BuilderToolbar.cs:287)
6. Unity.UI.Builder.BuilderCommandHandler:OnSaveDocument (UnityEngine.UIElements.KeyUpEvent) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Utilities/BuilderCommandHandler.cs:147)
7. UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

Code (CSharp):

1. Assets/Scripts/AnimationSelectionTree.uxml (5,14): Semantic - Unknown template name 'Button'
2. UnityEditor.AssetDatabase:Refresh ()
3. Unity.UI.Builder.BuilderDocumentOpenUXML:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Document/BuilderDocumentOpenUXML.cs:475)
4. Unity.UI.Builder.BuilderDocument:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Document/BuilderDocument.cs:271)
5. Unity.UI.Builder.BuilderToolbar:SaveDocument (bool) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Toolbar/BuilderToolbar.cs:287)
6. Unity.UI.Builder.BuilderCommandHandler:OnSaveDocument (UnityEngine.UIElements.KeyUpEvent) (at Library/PackageCache/com.unity.ui.builder@1.0.0-preview.11/Editor/Builder/Utilities/BuilderCommandHandler.cs:147)
7. UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

After that the buttons get deleted and I need to re-add them. It only happens occasionally, the rest of the time the file saves just fine.

Button template:

Code (CSharp):

1. <ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements" xsi="http://www.w3.org/2001/XMLSchema-instance" engine="UnityEngine.UIElements" editor="UnityEditor.UIElements" noNamespaceSchemaLocation="../../../../../../UIElementsSchema/UIElements.xsd" editor-extension-mode="False">
2.     <Style src="Button.uss" />
3.     <ui:Button text="Button" display-tooltip-when-elided="false" name="Button" class="button" />
4. </ui:UXML>

UI Builder generated template:

Code (CSharp):

1. <ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements" xmlns="UnityEngine.UIElements" xsi="http://www.w3.org/2001/XMLSchema-instance" editor-extension-mode="False">
2.     <ui:Template name="Button" src="/Packages/com.xvr.ui/Core/Button.uxml" />
3.     <ui:VisualElement name="rootContainer">
4.         <ui:VisualElement name="TabContainer" style="height: 41px; justify-content: flex-start; align-items: stretch; flex-direction: row; background-color: rgb(25, 25, 24);">
5.             <ui:Instance template="Button" name="Tab1" />
6.             <ui:Instance template="Button" name="Tab2" />
7.         </ui:VisualElement>
8.         <ui:VisualElement name="TabBody1" style="height: 336px; background-color: rgb(25, 25, 24); border-left-color: rgb(68, 72, 73); border-right-color: rgb(68, 72, 73); border-top-color: rgb(68, 72, 73); border-bottom-color: rgb(68, 72, 73); border-top-width: 2px;">
9.             <ui:ListView focusable="true" name="ScrollView" style="height: 385px;" />
10.         </ui:VisualElement>
11.     </ui:VisualElement>
12. </ui:UXML>

 

OP here.

I found one consistent way of reproducing the error and submitted a bug report. Casenr: 1300134

I wasn't using the sub-document functionality, this is all in the main document.

 

I just had this happen in 2020.3.21f1, Win 10