In Particle System, after enabling sub-emitter option when multiple Sub-Emitter components added in the list resulting NullReferenceException is thrown in the console. refer attached video

Steps to repro:
1. Create a new project
2. Hierarchy > right-click > Effects > Particle System
3. Inspector > Particle System > Enable Sub Emitters
4. Sub Emitters >Add Component in List

Actual Result:
NullReferenceException is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2020.2.0a20

Working Fine in:
2020.2.0a19, 2020.1.1f1, 2019.4.6f1.

Environment:
Windows 10 and macOS 10.15

Stack Trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.SubModuleUI.OnInspectorGUI (UnityEditor.InitialModuleUI initial) (at /Users/builduser/buildslave/unity/build/Modules/ParticleSystemEditor/ParticleSystemModules/SubModuleUI.cs:318)
UnityEditor.ParticleSystemUI.OnGUI (System.Single width, System.Boolean fixedWidth) (at /Users/builduser/buildslave/unity/build/Modules/ParticleSystemEditor/ParticleSystemUI.cs:189)
UnityEditor.ParticleEffectUI.InspectorParticleSystemGUI () (at /Users/builduser/buildslave/unity/build/Modules/ParticleSystemEditor/ParticleEffectUI.cs:863)
UnityEditor.ParticleEffectUI.OnGUI () (at /Users/builduser/buildslave/unity/build/Modules/ParticleSystemEditor/ParticleEffectUI.cs:1084)
UnityEditor.ParticleSystemInspector.OnInspectorGUI () (at /Users/builduser/buildslave/unity/build/Modules/ParticleSystemEditor/ParticleSystemEditor.cs:268)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass58_0.<CreateIMGUIInspectorFromEditor>b__0 () (at /Users/builduser/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Editor/Inspector/InspectorElement.cs:538)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:189)