fix: Select Distinct Column Throws null error

[Zhou-Ziheng]

fixes #1100

[codecov]

Codecov Report

Merging #1101 (7eb937c) into main (073f8f5) will increase coverage by 0.92%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1101      +/-   ##
==========================================
+ Coverage   41.98%   42.91%   +0.92%     
==========================================
  Files         430      434       +4     
  Lines       32342    32592     +250     
  Branches     8125     8199      +74     
==========================================
+ Hits        13578    13986     +408     
+ Misses      18711    18557     -154     
+ Partials       53       49       -4     

Flag	Coverage Δ
unit	42.91% <100.00%> (+0.92%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/iris-grid/src/GotoRow.tsx	36.70% <100.00%> (-0.80%)	⬇️
...shboard-core-plugins/src/panels/MockFileStorage.ts	14.81% <0.00%> (-10.19%)	⬇️
packages/iris-grid/src/IrisGridMetricCalculator.ts	70.58% <0.00%> (-9.42%)	⬇️
packages/grid/src/MockTreeGridModel.ts	89.09% <0.00%> (-5.09%)	⬇️
packages/iris-grid/src/IrisGridProxyModel.ts	67.05% <0.00%> (-1.85%)	⬇️
...ode-studio/src/styleguide/MockIrisGridTreeModel.ts	34.75% <0.00%> (-1.82%)	⬇️
packages/iris-grid/src/IrisGridRenderer.ts	24.36% <0.00%> (-0.49%)	⬇️
...id/src/mouse-handlers/GridSeparatorMouseHandler.ts	16.99% <0.00%> (-0.46%)	⬇️
packages/iris-grid/src/IrisGrid.tsx	27.36% <0.00%> (-0.39%)	⬇️
packages/iris-grid/src/IrisGridUtils.ts	51.57% <0.00%> (-0.09%)	⬇️
... and 39 more

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