Check for null MainView on UWP

[jamesmontemagno]

Description of Change

Additional null checks on main window if called at the start of an app.

Bugs Fixed

• Related to issue Evaluate throwing if Window is null on UWP #510

Provide links to issues here. Ensure that a GitHub issue was created for your feature or bug fix before sending PR.

API Changes

List all API changes here (or just put None), example:

Added:

• string Class.Property { get; set; }
• void Class.Method();

Changed:

• object Cell.OldPropertyName => object Cell.NewPropertyName

If there is an entirely new API, then you can use a more verbose style:

public static class NewClass {
    public static int SomeProperty { get; set; }
    public static void SomeMethod(string value);
}

Behavioral Changes

Describe any non-bug related behavioral changes that may change how users app behaves when upgrading to this version of the codebase.

PR Checklist

• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Updated documentation (see walkthrough)

[ghost]

Docs Build status updates of commit 459ec7b:

✅ Validation status: passed

File	Status	Preview URL	Details
Xamarin.Essentials/MainThread/MainThread.uwp.cs	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[ghost]

Docs Build status updates of commit 06f4e59:

✅ Validation status: passed

File	Status	Preview URL	Details
Xamarin.Essentials/MainThread/MainThread.uwp.cs	✅Succeeded

For more details, please refer to the build report.

Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report.

[newky2k]

Review, build and tested