fix accessing null object

[feiyun0112]

it will cause NullReferenceException

[codecov]

Codecov Report

Merging #5804 (45f0520) into main (7fafbf3) will increase coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #5804      +/-   ##
==========================================
+ Coverage   68.32%   68.38%   +0.06%     
==========================================
  Files        1131     1131              
  Lines      241291   241291              
  Branches    25053    25053              
==========================================
+ Hits       164863   165015     +152     
+ Misses      69923    69788     -135     
+ Partials     6505     6488      -17     

Flag	Coverage Δ
Debug	68.38% <0.00%> (+0.06%)	⬆️
production	63.01% <0.00%> (+0.07%)	⬆️
test	89.26% <ø> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/Microsoft.Data.Analysis/DataFrame.cs	86.40% <0.00%> (ø)
....Data/Evaluators/MultiOutputRegressionEvaluator.cs	0.37% <0.00%> (ø)
...ta/Evaluators/MulticlassClassificationEvaluator.cs	72.45% <0.00%> (ø)
....ML.Data/Evaluators/QuantileRegressionEvaluator.cs	67.84% <0.00%> (ø)
...forms/FeatureContributionCalculationTransformer.cs	78.04% <0.00%> (ø)
src/Microsoft.ML.Maml/MAML.cs	24.75% <0.00%> (-1.46%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.67% <0.00%> (+0.14%)	⬆️
...StandardTrainers/Standard/LinearModelParameters.cs	66.32% <0.00%> (+0.25%)	⬆️
src/Microsoft.ML.Data/Utils/LossFunctions.cs	67.35% <0.00%> (+0.51%)	⬆️
...oft.ML.StandardTrainers/Standard/SdcaMulticlass.cs	92.49% <0.00%> (+1.02%)	⬆️
... and 8 more

[michaelgsharp]

LGTM