[Bug]: ObservableMixin requires full initialization

[Nextra]

Describe the bug 🐞

In #4257 ObservableMixins.cs had a static constructor added, which enforces ReactiveUI to be fully initialized. This leads to innocuous helpers like ObservableMixins.WhereNotNull to suddenly break if used before RxAppBuilder.BuildApp is called.

As far as I can tell none of the helpers in ObservableMixins require initialization, but I am not sure if other Mixins (especially those in platform-specific modules) are affected.

Step to reproduce

1. Use ObservableMixins.WhereNotNull before ReactiveUI initialization
2. Exception is thrown

Reproduction repository

No response

Expected behavior

Mixins that provide functionality not tied to ReactiveUI features should not require app initialization.

Screenshots 🖼️

No response

IDE

Visual Studio 2022

Operating system

No response

Version

No response

Device

No response

ReactiveUI Version

23.2.27

Additional information ℹ️

No response

[glennawatson]

I'll work on a fix for this one in the next couple days. Thanks for reporting. Fix will be likely moving out the extension methods that require the state (or the other way around).

[Nextra]

I suspected there might be a few more complex cases. Thanks for your work!