How to reproduce:
1. Open the project "WordWrappedLabel.zip"
2. Go to Window -> My Window and dock it somewhere
3. In the Project window open Editor/TestEditorScript.cs
4. In the TestEditorScript.cs comment out EditorGUILayout.EndToggleGroup(); line and save it
5. Focus the Editor and observe the error in the Console
6. Uncomment the same line and focus the Editor

Expected result: the error is no longer thrown once the line is uncommented
Actual result: the error remains in the Console

Reproducible with: 2019.2.0a1, 2019.2.21f1, 2019.3.16f1, 2019.4.6f1, 2020.1.0f1, 2020.2.0a19
Could not test with: 2018.4.26f1, 2019.1.0a14, 2019.1.14f1

Notes:
1. Full error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorStyles.get_wordWrappedLabel () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/EditorStyles.cs:41)
TestEditorScript..ctor () (at Assets/Editor/TestEditorScript.cs:6)

2. Versions 2018.4.26f1, 2019.1.14f1 do not throw any NullReferenceExceptions (when the line is commented and when the line is uncommented)