Check if Tabs are null in SetDefaultActiveTabAsync

[DrMueller]

Describe the bug
SetDefaultActiveTabAsync accesses the tabs-field, without checking for null. As DisposeAsyncCore does set the tabs to null, this causes a NullReference-exception in cases of exceptions (or if someone would navigate aways fast I guess).

To Reproduce
Steps to reproduce the behavior:

1. Create a tabs component
2. Create a content component within the tab
3. Have the content component throw an exception on OnInitializedAsync

Expected behavior
Tabs component stops gracefully

Screenshots

Versions (please complete the following information):

• .NET Version: .NET8
• BlazorBootstrap: 3.4.0
• Blazor WebAssembly / Server: Server
• Blazor Interactive Render Mode: Server
• Blazor Interactivity Location: Global