How to reproduce:
1. Create or open any project
2. Open Preferences window
3. Tick any checkbox in any tab

Expected results: No errors are thrown in the console
Actual results: NullReferenceException in UnityEditor.Android.SDKManager

Reproducible with: 2019.2.6f1
Not reproducible with: 2017.4.32f1, 2018.4.8f1, 2019.3.0b4, 2020.1.0a4

Full error description:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Android.SDKManager..ctor () (at <1b1a61ffd3924442960b571323440781>:0)
UnityEditor.Android.SDKManager.GetInstance () (at <1b1a61ffd3924442960b571323440781>:0)
UnityEditor.Android.AndroidPreferenceWindowExtension.WritePreferences () (at <1b1a61ffd3924442960b571323440781>:0)
UnityEditor.PreferencesProvider.WritePreferences () (at /home/builduser/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesSettingsProviders.cs:820)
UnityEditor.PreferencesProvider.ApplyChangesToPrefs (System.Boolean force) (at /home/builduser/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesSettingsProviders.cs:531)
UnityEditor.PreferencesProvider.ShowExternalApplications (System.String searchContext) (at /home/builduser/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesSettingsProviders.cs:375)
UnityEditor.PreferencesProvider.OnGUI (System.String searchContext, System.Action`1[T] drawAction) (at /home/builduser/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesSettingsProviders.cs:308)
UnityEditor.PreferencesProvider+<CreateExternalToolsProvider>c__AnonStorey1.<>m__0 (System.String searchContext) (at /home/builduser/buildslave/unity/build/Editor/Mono/PreferencesWindow/PreferencesSettingsProviders.cs:208)
UnityEditor.SettingsProvider.OnGUI (System.String searchContext) (at /home/builduser/buildslave/unity/build/Editor/Mono/Settings/SettingsProvider.cs:103)
UnityEditor.SettingsWindow.DrawControls () (at /home/builduser/buildslave/unity/build/Editor/Mono/Settings/SettingsWindow.cs:297)
UnityEditor.SettingsWindow.DrawSettingsPanel () (at /home/builduser/buildslave/unity/build/Editor/Mono/Settings/SettingsWindow.cs:288)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at /home/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:279)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at /home/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:465)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e) (at /home/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:448)
UnityEngine.UIElements.IMGUIContainer.HandleEvent (UnityEngine.UIElements.EventBase evt) (at /home/builduser/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:428)
UnityEngine.UIElements.MouseCaptureDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at /home/builduser/buildslave/unity/build/Modules/UIElements/Events/MouseCaptureDispatchingStrategy.cs:87)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at /home/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:280)
UnityEngine.UIElements.EventDispatcher.Dispatch (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel, UnityEngine.UIElements.DispatchMode dispatchMode) (at /home/builduser/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:156)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at /home/builduser/buildslave/unity/build/Modules/UIElements/Panel.cs:189)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at /home/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:250)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at /home/builduser/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:68)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at /home/builduser/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)

Notes:
- Only happens if Android support is installed