How to reproduce:
1. Open attached project "Case_1223020"
2. Click Edit->Project Settings
3. Select "Localization" and expand "String Database"
4. Select a "Locale Selector" and remove it by clicking the "-"

Reproducible with: 0.5.0 (2019.3.3f1, 2020.1.0a25, 2020.1.0a1), 0.6.1-preview (2019.3.3f1, 2020.1.0a25, 2020.1.0a1)
Not reproducible with: 0.3.1-preview (2018.4.18f1), 0.4.0 (2019.3.3f1) (due to different Localization Project Settings)
Could not test with: 2017.4.37f1 (No package)

First lines of stack trace:
at <unknown> <0xffffffff>
at (wrapper managed-to-native) UnityEditor.SerializedProperty.NextVisibleInternal (UnityEditor.SerializedProperty,bool) \[0x00014] in <e859a479319a4a5a8d9711977e98d28e>:0
at UnityEditor.SerializedProperty.NextVisible (bool) \[0x0000b] in <e859a479319a4a5a8d9711977e98d28e>:0
at UnityEditor.PropertyHandler.GetHeight (UnityEditor.SerializedProperty,UnityEngine.GUIContent,bool) \[0x0011b] in <e859a479319a4a5a8d9711977e98d28e>:0
at UnityEditor.PropertyHandler.OnGUILayout (UnityEditor.SerializedProperty,UnityEngine.GUIContent,bool,UnityEngine.GUILayoutOption\[]) \[0x00047] in <e859a479319a4a5a8d9711977e98d28e>:0
at UnityEditor.EditorGUILayout.PropertyField (UnityEditor.SerializedProperty,UnityEngine.GUIContent,bool,UnityEngine.GUILayoutOption\[]) \[0x0000b] in <e859a479319a4a5a8d9711977e98d28e>:0