Steps to reproduce:
1. Create new Unity project
2. Create new UI Document
3. Open it in UI Builder
4. In Library section enable "Editor Extension Authoring"
5. Add Curve Element to the Hierarchy
6. Make sure "CurveField" element is selected in Hierarchy
7. In UI Builder > Inspector > Attributes select on Curve Value to open Curve Editor window
8. In the Curve Editor window double click on the Curve
9. Save changes with shortcut CRTL+S
10. Observe the Console window
 

Actual results: Multiple errors are thrown in the Console window

Expected results: No errors are thrown in the Console

Reproducible with versions: 6000.0.49f1, 6000.1.4f1, 6000.2.0b2

Tested on (OS): Windows 11

Errors:
*
{code:java}
ArgumentOutOfRangeException: [UxmlAttributeConverter] AnimationCurve's KeyFrame does not have the correct format.
Parameter name: value
UnityEditor.UIElements.AnimationCurveAttributeConverter.FromString (System.String value) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlAttributeConverter`1[T].UnityEditor.UIElements.IUxmlAttributeConverter.FromString (System.String value, UnityEngine.UIElements.CreationContext cc) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlAttributeConverter.TryConvertFromString (System.Type type, System.String value, UnityEngine.UIElements.CreationContext cc, System.Object& result) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlSerializedAttributeDescription.TryGetValueFromBagAsObject (UnityEngine.UIElements.IUxmlAttributes bag, UnityEngine.UIElements.CreationContext cc, System.Object& value) (at <314f8032995c4bb8a81979ad42f40d7d>:0){code}

*
{code:java}
Asset import failed, "Assets/test1.uxml" > ArgumentOutOfRangeException: [UxmlAttributeConverter] AnimationCurve's KeyFrame does not have the correct format.
Parameter name: value
UnityEditor.UIElements.AnimationCurveAttributeConverter.FromString (System.String value) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlAttributeConverter`1[T].UnityEditor.UIElements.IUxmlAttributeConverter.FromString (System.String value, UnityEngine.UIElements.CreationContext cc) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlAttributeConverter.TryConvertFromString (System.Type type, System.String value, UnityEngine.UIElements.CreationContext cc, System.Object& result) (at <314f8032995c4bb8a81979ad42f40d7d>:0)
UnityEditor.UIElements.UxmlSerializedAttributeDescription.TryGetValueFromBagAsObject (UnityEngine.UIElements.IUxmlAttributes bag, UnityEngine.UIElements.CreationContext cc, System.Object& value) (at <314f8032995c4bb8a81979ad42f40d7d>:0){code}

*
{code:java}
NullReferenceException: Object reference not set to an instance of an object
Unity.UI.Builder.BuilderDocumentOpenUXML.PostSaveToDiskChecksAndFixes (System.String newUxmlPath, System.Boolean needsFullRefresh) (at <520627d618084211ad7f451c0d732996>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.ReloadDocument (UnityEngine.UIElements.VisualElement documentRootElement, System.String newUxmlPath, System.Collections.Generic.List`1[T] savedUSSFiles) (at <520627d618084211ad7f451c0d732996>:0){code}