Enable nullability in TabPageCollectionEditor

[gpetrou]

Proposed changes

• Enable nullability in TabPageCollectionEditor.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 74.27250%. Comparing base (74caf32) to head (1e6e27d).
Report is 13 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11283         +/-   ##
===================================================
+ Coverage   74.26878%   74.27250%   +0.00371%     
===================================================
  Files           3025        3025                 
  Lines         626861      626870          +9     
  Branches       46742       46744          +2     
===================================================
+ Hits          465562      465592         +30     
+ Misses        157959      157935         -24     
- Partials        3340        3343          +3     

Flag	Coverage Δ
Debug	74.27250% <75.00000%> (+0.00371%)	⬆️
integration	18.00556% <0.00000%> (+0.00084%)	⬆️
production	46.99986% <75.00000%> (+0.00904%)	⬆️
test	96.99415% <ø> (ø)
unit	43.96795% <75.00000%> (+0.03231%)	⬆️

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[Epica3055]

looks good 😁

[LeafShi1]

Using is {} obj can more simpler

      if (TypeDescriptor.CreateInstance(host, itemType, argTypes: null, args: null) is { } obj)
      {
          return obj;
      }

      throw new InvalidOperationException(
          string.Format(
          SR.CollectionEditorCreateInstanceError,
          nameof(IDesignerHost),
          nameof(TypeDescriptor),
          itemType.FullName));