Enable nullability in some ControlDesigner members

[gpetrou]

Proposed changes

• Enable nullability in some ControlDesigner members.

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 34.88372% with 28 lines in your changes are missing coverage. Please review.

Project coverage is 74.27201%. Comparing base (74caf32) to head (b28ec89). Report is 18 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #11285         +/-   ##
===================================================
+ Coverage   74.26878%   74.27201%   +0.00322%     
===================================================
  Files           3025        3025                 
  Lines         626861      626866          +5     
  Branches       46742       46743          +1     
===================================================
+ Hits          465562      465586         +24     
+ Misses        157959      157937         -22     
- Partials        3340        3343          +3     

Flag	Coverage Δ
Debug	74.27201% <34.88372%> (+0.00322%)	:arrow_up:
integration	18.00581% <32.55814%> (+0.00109%)	:arrow_up:
production	46.99841% <34.88372%> (+0.00759%)	:arrow_up:
test	96.99415% <ø> (ø)
unit	43.96786% <18.60465%> (+0.03222%)	:arrow_up:

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[SimonZhao888]

LGTM!