ArgumentNullException error is thrown in Updating Unity Collaborate Package When open collaborate window.

Steps to repro:
1. Create a new project
2. Windows > Package Manager > Unity collaborate(1.3.7) > Install(If not installed)
3. Open collaborate Window
4. Windows > Package Manager > Up to date (2.1.0-preview.3 )

Actual Result:
ArgumentNullException error is thrown on Updating Unity Collaborate Package.

Expected Result:
No error in the console.

Reproducible in Package Version: 2.1.0-preview.3, 1.3.7.

Reproducible on:
2020.2.0a5, 2020.1.0b4, 2020.1.0a22.

Not Reproducible on
2020.1.0a21, 2019.3.7f1.

Environment:
Windows 10 & Mac 10.15.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: styleSheet
UnityEngine.UIElements.VisualElementStyleSheetSet.Add (UnityEngine.UIElements.StyleSheet styleSheet) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/VisualElementStyleSheetSet.cs:31)
Unity.Cloud.Collaborate.Components.IconButton.InvokeGenerateVisualContent (UnityEngine.UIElements.MeshGenerationContext mgc) (at library/packagecache/com.unity.collab-proxy@2.1.0-preview.3/editor/components/iconbutton.cs:31)
Unity.Cloud.Collaborate.Components.IconButton.set_generateVisualContent (System.Action`1[T] value) (at library/packagecache/com.unity.collab-proxy@2.1.0-preview.3/editor/components/iconbutton.cs:24)
System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceMono (System.Boolean nonPublic) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceSlow (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceDefaultCtor (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.Activator.CreateInstance[T] () (at <437ba245d8404784b9fbab9b439ac908>:0)
UnityEngine.UIElements.UxmlFactory`2[TCreatedType,TTraits].Create (UnityEngine.UIElements.IUxmlAttributes bag, UnityEngine.UIElements.CreationContext cc) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/UxmlFactory.cs:183)
UnityEngine.UIElements.VisualTreeAsset.Create (UnityEngine.UIElements.VisualElementAsset asset, UnityEngine.UIElements.CreationContext ctx) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:537)
UnityEngine.UIElements.VisualTreeAsset.CloneSetupRecursively (UnityEngine.UIElements.VisualElementAsset root, System.Collections.Generic.Dictionary`2[TKey,TValue] idToChildren, UnityEngine.UIElements.CreationContext context) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:311)
UnityEngine.UIElements.VisualTreeAsset.CloneSetupRecursively (UnityEngine.UIElements.VisualElementAsset root, System.Collections.Generic.Dictionary`2[TKey,TValue] idToChildren, UnityEngine.UIElements.CreationContext context) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:354)
UnityEngine.UIElements.VisualTreeAsset.CloneSetupRecursively (UnityEngine.UIElements.VisualElementAsset root, System.Collections.Generic.Dictionary`2[TKey,TValue] idToChildren, UnityEngine.UIElements.CreationContext context) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:354)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target, System.Collections.Generic.Dictionary`2[TKey,TValue] slotInsertionPoints, System.Collections.Generic.List`1[T] attributeOverrides) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:292)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target, System.Int32& firstElementIndex, System.Int32& elementAddedCount) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:229)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:218)
Unity.Cloud.Collaborate.Components.TopBar.get_Item (System.Int32 key) (at library/packagecache/com.unity.collab-proxy@2.1.0-preview.3/editor/components/topbar.cs:43)
System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoCMethod.InternalInvoke (System.Object obj, System.Object[] parameters) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceMono (System.Boolean nonPublic) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceSlow (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.RuntimeType.CreateInstanceDefaultCtor (System.Boolean publicOnly, System.Boolean skipCheckThis, System.Boolean fillCache, System.Threading.StackCrawlMark& stackMark) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.Activator.CreateInstance[T] () (at <437ba245d8404784b9fbab9b439ac908>:0)
UnityEngine.UIElements.UxmlFactory`2[TCreatedType,TTraits].Create (UnityEngine.UIElements.IUxmlAttributes bag, UnityEngine.UIElements.CreationContext cc) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/UxmlFactory.cs:183)
UnityEngine.UIElements.VisualTreeAsset.Create (UnityEngine.UIElements.VisualElementAsset asset, UnityEngine.UIElements.CreationContext ctx) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:537)
UnityEngine.UIElements.VisualTreeAsset.CloneSetupRecursively (UnityEngine.UIElements.VisualElementAsset root, System.Collections.Generic.Dictionary`2[TKey,TValue] idToChildren, UnityEngine.UIElements.CreationContext context) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:311)
UnityEngine.UIElements.VisualTreeAsset.CloneSetupRecursively (UnityEngine.UIElements.VisualElementAsset root, System.Collections.Generic.Dictionary`2[TKey,TValue] idToChildren, UnityEngine.UIElements.CreationContext context) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:354)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target, System.Collections.Generic.Dictionary`2[TKey,TValue] slotInsertionPoints, System.Collections.Generic.List`1[T] attributeOverrides) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:292)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target, System.Int32& firstElementIndex, System.Int32& elementAddedCount) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:229)
UnityEngine.UIElements.VisualTreeAsset.CloneTree (UnityEngine.UIElements.VisualElement target) (at /Users/builduser/buildslave/unity/build/Modules/UIElements/UXML/VisualTreeAsset.cs:218)
Unity.Cloud.Collaborate.Views.MainPageView..ctor () (at library/packagecache/com.unity.collab-proxy@2.1.0-preview.3/editor/views/mainpageview.cs:45)
Unity.Cloud.Collaborate.UserInterface.CollaborateWindow.OnEnable () (at library/packagecache/com.unity.collab-proxy@2.1.0-preview.3/editor/userinterface/collaboratewindow.cs:118)