NullReferenceException errors thrown continuously on selecting TSS Theme file preset asset.

Steps to repro:
1. Create a new Project.
2. Project Window > Click on "+" > UI Toolkit >
3. TSS Theme File > Create its preset asset.
4. Select newly created preset asset.

Actual Result:
NullReferenceException errors in the console.

Expected Result:
No errors in the console.

Reproducible in:
2021.1.0a9, 2020.2.0f1, 2020.2.0b11.

Note:
-Unable to test on 2020.2.b10 to 2020.2.0a1 due to errors on installing UI builder package.
-TSS file Theme option is not available in 2020.1.1f1

Environment:
Occurring on Windows 10 & Mac 10.15.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.StyleSheets.ThemeStyleSheetImporterEditor.LoadThemeAssetDefinitionState (UnityEditor.UIElements.StyleSheets.ThemeAssetDefinitionState extraData, System.String assetPath) (at <7ba5bac7360d4ac8a9ebd5b4256fdfe2>:0)
UnityEditor.UIElements.StyleSheets.ThemeStyleSheetImporterEditor.InitializeExtraDataInstance (UnityEngine.Object extraData, System.Int32 targetIndex) (at <7ba5bac7360d4ac8a9ebd5b4256fdfe2>:0)
UnityEditor.AssetImporters.AssetImporterEditor.InternalSetTargets (UnityEngine.Object[] t) (at <a19e86c7bf354115a4c6a5953c0575cb>:0)
UnityEditor.Presets.PresetEditor:OnEnable()