Fix 12571 "DisplayMember" property cannot expand

[LeafShi1]

Fixes #12571

Proposed changes

• When DataSource is null, do not return value immediately but return after the designBindingPicker is created.

Customer Impact

• The dropdown list of the DisplayMember property of the PropertyGrid can be expanded when DataSource is null

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

There no "Open" button for the "DiaplayMember" property and cannot not expand the dropdown list in the PropertyGrid control

After

The dropdown list of the DisplayMember property can be expanded when DataSource is null

AfterChange.mp4

Test methodology

• Manually

Test environment(s)

• .net 10.0.0-beta.25418.105

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.12339%. Comparing base (8818780) to head (ae44654).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13833         +/-   ##
===================================================
+ Coverage   77.10728%   77.12339%   +0.01610%     
===================================================
  Files           3273        3273                 
  Lines         644882      644877          -5     
  Branches       47688       47687          -1     
===================================================
+ Hits          497251      497351        +100     
+ Misses        143958      143857        -101     
+ Partials        3673        3669          -4     

Flag	Coverage Δ
Debug	77.12339% <0.00000%> (+0.01610%)	⬆️
integration	18.99213% <0.00000%> (+0.00694%)	⬆️
production	51.95931% <0.00000%> (+0.03564%)	⬆️
test	97.41248% <ø> (ø)
unit	49.37260% <0.00000%> (+0.02655%)	⬆️

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[ricardobossan]

I've tested and couldn't reproduce the issue anymore. All LGTM!