How to reproduce:
1. Create new Unity project
2. File > Build Settings > Player Settings > Player > Other Settings
3. Define any Scripting Symbol
4. Switch platforms
5. Click "Player Settings"

Expected results: No exceptions are thrown
Actual results: InvalidCastException and NullReferenceException thrown

Reproducible with: 2020.2.0b3, 2020.2.0b8
Not reproducible with: 2018.4.28f1, 2019.4.13f1, 2020.1.10f1, 2020.2.0b2

Full errors:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.PlayerSettingsEditor.UpdateScriptingDefineSymbolsLists () (at <b39c8a57494f4bd1bf11d6afae39993d>:0)
UnityEditor.PlayerSettingsEditor.CheckConsistency (UnityEditor.BuildTargetGroup targetGroup) (at <b39c8a57494f4bd1bf11d6afae39993d>:0)

InvalidCastException: Specified cast is not valid.
UnityEditor.EditorGUILayout.BeginScrollView (UnityEngine.Vector2 scrollPosition, System.Boolean alwaysShowHorizontal, System.Boolean alwaysShowVertical, UnityEngine.GUIStyle horizontalScrollbar, UnityEngine.GUIStyle verticalScrollbar, UnityEngine.GUIStyle background, UnityEngine.GUILayoutOption[] options) (at <b39c8a57494f4bd1bf11d6afae39993d>:0)