On changing properties in FBX ExporterSettings Preset, NullReferenceException is thrown in the console, refer the attached video.

Steps to repro:
1. Create a new Project
2. Windows > Package Manager > FBX Exporter > Install
3. Edit > Project Settings > FBX Expor
4. Create preset of FBX Export Setting
5. Project > FBX Export Setting Preset > Inspector > Keep Qpen/Hide Native Menu > Disable/Enable

Actual Result:
NullReferenceException is thrown on changing properties in FBX ExporterSettings Preset

Expected Result:
No Error in the console

Tested with Unity versions:
2020.2.0a1, 2020.1.0a26, 2019.3.3f1, 2019.1.0a9

Package found version: preview.4 - 3.0.1, preview.2 - 3.0.0 preview.2 - 2.0.1

Environment:
Windows10 and macOS 10.14

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Formats.Fbx.Exporter.ExportSettings.Save () (at Library/PackageCache/com.unity.formats.fbx@3.0.1-preview.2/Editor/FbxExportSettings.cs:1378)
UnityEditor.Formats.Fbx.Exporter.ExportSettingsEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.formats.fbx@3.0.1-preview.2/Editor/FbxExportSettings.cs:186)
UnityEditor.Presets.PresetEditor.DrawInternalInspector () (at <e859a479319a4a5a8d9711977e98d28e>:0)
UnityEditor.Presets.PresetEditor.DrawPresetData () (at <e859a479319a4a5a8d9711977e98d28e>:0)
UnityEditor.Presets.PresetEditor.OnInspectorGUI () (at <e859a479319a4a5a8d9711977e98d28e>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <e859a479319a4a5a8d9711977e98d28e>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)