Enable nullability for `ToolStripTemplateNode`

[LeafShi1]

Enable nullability for ToolStripTemplateNode

Microsoft Reviewers: Open in CodeFlow

[Tanya-Solyanik]

@LeafShi1 - this PR got labeled as 'area: NRT' by 2 tools, by https://github.com/dotnet/winforms/blob/main/.github/policies/resourceManagement.yml and by https://github.com/dotnet/winforms/blob/main/.github/workflows/labeler-predict-pulls.yml, could you please remove the rule that adds this label from the resourceManagement bot? Also, going forward we should use area- prefix because labeler works with it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 4.22535% with 136 lines in your changes missing coverage. Please review.

Project coverage is 62.72004%. Comparing base (a345d2c) to head (cb9f820). Report is 127 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13401         +/-   ##
===================================================
- Coverage   62.72868%   62.72004%   -0.00864%     
===================================================
  Files           1561        1562          +1     
  Lines         159997      160027         +30     
  Branches       14918       14960         +42     
===================================================
+ Hits          100364      100369          +5     
- Misses         58856       58883         +27     
+ Partials         777         775          -2     

Flag	Coverage Δ
Debug	62.72004% <4.22535%> (-0.00864%)	:arrow_down:
integration	11.33488% <0.00000%> (+0.00168%)	:arrow_up:
production	40.91164% <4.22535%> (-0.00756%)	:arrow_down:
test	95.70296% <ø> (ø)
unit	38.31541% <4.22535%> (-0.00778%)	:arrow_down:

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[Tanya-Solyanik]

@LeafShi1 - could you please factor out the bot fix to merge it ASAP? I have concerns about the nullability changes, I feel that site or DesignHost being null is an indication of a bigger problem - template node is a design time element and it should not be instantiated outside the designer. I need to investigate this scenario, i.e. I guess that inproc designer would throw a null reference exception in this case.

[LeafShi1]

@LeafShi1 - could you please factor out the bot fix to merge it ASAP? I have concerns about the nullability changes, I feel that site or DesignHost being null is an indication of a bigger problem - template node is a design time element and it should not be instantiated outside the designer. I need to investigate this scenario, i.e. I guess that inproc designer would throw a null reference exception in this case.

Revert the changes in resourceManagement.yml and put it in a separate PR https://github.com/dotnet/winforms/pull/13417

[KlausLoeffelmann]

Can we wait with this, until the DarkMode changes are merged? I cannot remember to have touched this file in particular, but I touched so many toolstrip realted files, it's probably best to wait, until we have merged that.

Thanks!