How to reproduce:
1. Open the attached "VariableSwap" project
2. Create a new UI Document (in the project window right-click->Create->UI Toolkit->UI Document) and open it
3. From the UI Builder Library add two VisualElement and one Button elements
4. Move the added Button to be a child of the VisualElement which is first in the Hierarchy
5. In the UI Builder Hierarchy select the Button
6. In the UI Builder Inspector expand the Border section
7. In the UI Builder Inspector change the Width to 5px
8. Save the UI Document (in the Viewport File->Save)

Expected results: No errors are thrown, the changed variable does not get switched with the element next in the Hierarchy
Actual results: The "Accessing invalid property" error is thrown, the changed variables on the Button get switched with the second VisualElement

Reproducible with: 1.0.0-preview.5, 1.0.0-preview.9 (2019.4.13f1, 2020.1.10f1, 2020.2.0b8, 2021.1.0a2)
Not reproducible with: 1.0.0-preview.4 (2019.4.13f1, 2020.1.10f1, 2020.2.0b8, 2021.1.0a2)
Could not test with: 2018.4.28f1 because the UI Builder package was not yet introduced

Notes:
- Only the variables that were changed before saving are swapped
- The variables get swapped between the child and the next in the Hierarchy that is at the same level as the child's parent
- Sometimes saving the UI Document before the last step will prevent the issue from reproducing