Reproduction steps:
1. Open the Auto Showroom Template
2. Select Showroom > LightmapSwitcher in the Hierarchy
3. Expand Settings in the Inspector

Expected result: There are no errors
Actual result: There's an error in the Themes Header

Reproducible with: 2020.3.5f
Not reproducible with: 2019.4.24f1 (The error is not exposed, but will appear if Project is updated to 2020.3)
Could not test with: 2021.1, 2021.2 (No Auto Showroom Template)

Workaround:
Comment the line 50:
//toggleSettingGroup = EditorGUILayout.BeginFoldoutHeaderGroup(toggleSettingGroup, "Settings");
and the line 91:
//EditorGUILayout.EndFoldoutHeaderGroup();
in LevelLightmapDataEditor.cs