[BUG] VMSS extension can not be created as part of the VMSS Creation request

[desokey]

Library name and version

Azure.ResourceManager.Compute

Describe the bug

When creating new VMSS using CreateOrUpdate method , and adding CustomScript extension as part of VirtualMachinecaleetData object ,
Creation faild

Expected behavior

Successful Creation

Actual behavior

Object reference not set to an instance of an object. , it complians about a null collection,

at Azure.ResourceManager.Compute.VirtualMachineScaleSetExtensionData.Azure.Core.IUtf8JsonSerializable.Write(Utf8JsonWriter writer)
at Azure.ResourceManager.Compute.Models.VirtualMachineScaleSetExtensionProfile.Azure.Core.IUtf8JsonSerializable.Write(Utf8JsonWriter writer)
at Azure.Core.Utf8JsonWriterExtensions.WriteObjectValue(Utf8JsonWriter writer, IUtf8JsonSerializable value)
at Azure.ResourceManager.Compute.Models.VirtualMachineScaleSetVmProfile.Azure.Core.IUtf8JsonSerializable.Write(Utf8JsonWriter writer)
at Azure.ResourceManager.Compute.VirtualMachineScaleSetData.Azure.Core.IUtf8JsonSerializable.Write(Utf8JsonWriter writer)
at Azure.Core.Utf8JsonWriterExtensions.WriteObjectValue(Utf8JsonWriter writer, IUtf8JsonSerializable value)
at Azure.ResourceManager.Compute.VirtualMachineScaleSetsRestOperations.CreateCreateOrUpdateRequest(String virtualMachineScaleSetName, String subscriptionId, String resourceGroupName, VirtualMachineScaleSetData data)
at Azure.ResourceManager.Compute.VirtualMachineScaleSetsRestOperations.d__9.MoveNext()
at Azure.ResourceManager.Compute.VirtualMachineScaleSetCollection.d__5.MoveNext()

Reproduction Steps

vmssModel.ExtensionProfile = new VirtualMachineScaleSetExtensionProfile()
                        {
                            Extensions =
                           {
                                 new VirtualMachineScaleSetExtensionData("CustomScript")
                                {
                                       AutoUpgradeMinorVersion = true,
                                       EnableAutomaticUpgrade = false,
                                       Settings = BinaryData.FromObjectAsJson(new
                                          {}),
                                       ProvisionAfterExtensions= {},
                                       ProtectedSettings = BinaryData.FromObjectAsJson(new
                                        {
                                               
                                        }),
                                       Publisher = "Microsoft.Azure.Extensions",
                                       ExtensionType = "CustomScript",
                                       TypeHandlerVersion = "2.1"
                                 }
                           }
                        },

Environment

Dotnet

[azure-sdk]

Label prediction was below confidence level 0.6 for Model:ServiceLabels: 'Storage:0.3133556,Azure.Identity:0.057873018,Service Bus:0.047534425'

[desokey]

Just to save the team some time , here is my investigation so far
1- The Request complains about the ProvisionAfterExtensions property being null, if you skip this check , you will face another issue , it will complain about the name of the extension is not provided although it is provided in the payload but the Writer does not write the name property in this method Azure.ResourceManager.Compute.Models.VirtualMachineScaleSetExtensionProfile.Azure.Core.IUtf8JsonSerializable.Write