To reproduce:
1. Open any project
2. Go toBuild Settings > Player Settings > Other settings (any alternative element list will do)
3. Locate The Color Gamut For Mac parameter
4. Try dragging the sRGB dragging handle

Result: dragging produces console errors regarding NullReferenceException, elements cannot be switched places

Reproduced in: 2019.2.0a12, 2019.2.0a13, 2019.3.0a1
Not reproduced in: 2018.3.14f1, 2017.4.26f1

Note: full error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditorInternal.ReorderableList.DoDraggingAndSelection (UnityEngine.Rect listRect) (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:708)
UnityEditorInternal.ReorderableList.DoListElements (UnityEngine.Rect listRect) (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:595)
UnityEditorInternal.ReorderableList.DoLayoutList () (at C:/buildslave/unity/build/Editor/Mono/GUI/ReorderableList.cs:396)
UnityEditor.PlayerSettingsEditor.ColorGamutGUI (UnityEditor.BuildTargetGroup targetGroup) (at C:/buildslave/unity/build/Editor/Mono/Inspector/PlayerSettingsEditor/PlayerSettingsEditor.cs:1351)
UnityEditor.PlayerSettingsEditor.OtherSectionRenderingGUI (UnityEditor.Build.BuildPlatform platform, UnityEditor.BuildTargetGroup targetGroup, UnityEditor.Modules.ISettingEditorExtension settingsExtension) (at C:/buildslave/unity/build/Editor/Mono/Inspector/PlayerSettingsEditor/PlayerSettingsEditor.cs:1512)
UnityEditor.PlayerSettingsEditor.OtherSectionGUI (UnityEditor.Build.BuildPlatform platform, UnityEditor.BuildTargetGroup targetGroup, UnityEditor.Modules.ISettingEditorExtension settingsExtension, System.Int32 sectionIndex) (at C:/buildslave/unity/build/Editor/Mono/Inspector/PlayerSettingsEditor/PlayerSettingsEditor.cs:1447)
UnityEditor.PlayerSettingsEditor.OnInspectorGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/PlayerSettingsEditor/PlayerSettingsEditor.cs:577)
UnityEditor.AssetSettingsProvider.OnGUI (System.String searchContext) (at C:/buildslave/unity/build/Editor/Mono/Settings/Providers/AssetSettingsProvider.cs:94)
UnityEditor.SettingsWindow.DrawControls () (at C:/buildslave/unity/build/Editor/Mono/Settings/SettingsWindow.cs:322)
UnityEditor.SettingsWindow.DrawSettingsPanel () (at C:/buildslave/unity/build/Editor/Mono/Settings/SettingsWindow.cs:313)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:298)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:493)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:476)
UnityEngine.UIElements.IMGUIContainer.SendEventToIMGUI (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:466)
UnityEngine.UIElements.IMGUIContainer.HandleEvent (UnityEngine.UIElements.EventBase evt) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:447)
UnityEngine.UIElements.MouseCaptureDispatchingStrategy.DispatchEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/Events/MouseCaptureDispatchingStrategy.cs:93)
UnityEngine.UIElements.EventDispatcher.ProcessEvent (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.IPanel panel) (at C:/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 C:/buildslave/unity/build/Modules/UIElements/EventDispatcher.cs:156)
UnityEngine.UIElements.BaseVisualElementPanel.SendEvent (UnityEngine.UIElements.EventBase e, UnityEngine.UIElements.DispatchMode dispatchMode) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:189)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:255)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:78)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)