ML.Ranking LightGBM - Getting error "Value cannot be null. Parameter name: items"

### System information

- Windows:
- .net Core 3.0:

### Issue

- I am trying to generate a simple ranking of candidates based on a few features for a recruitment application.
- But when running the trainer, I get the following message which is not clear - "Value cannot be null. Parameter name: items"

### Source code / logs


```C#
//Training Pipeline
IEstimator dataPipeline = mlContext.Transforms.Categorical.OneHotEncoding("HIGHESTEDUCATION", "HIGHESTEDUCATION")
.Append(mlContext.Transforms.Categorical.OneHotEncoding("SOURCE", "SOURCE"))
.Append(mlContext.Transforms.Text.FeaturizeText("SKILLSET", "SKILLSET"))
.Append(mlContext.Transforms.Categorical.OneHotEncoding("TOWNCITY", "TOWNCITY"))
.Append(mlContext.Transforms.Categorical.OneHotEncoding("YEARSOFEXPERIENCE", "YEARSOFEXPERIENCE"))
.Append(mlContext.Transforms.Concatenate("Features", "HIGHESTEDUCATION", "SKILLSET", "SOURCE", "TOWNCITY", "YEARSOFEXPERIENCE"))
.Append(mlContext.Transforms.Conversion.MapValueToKey("Label","Label"))
.Append(mlContext.Transforms.Conversion.Hash("GroupId", nameof(Candidate.VACANCYID), numberOfBits: 20));

// Set the LightGBM LambdaRank trainer.
IEstimator trainer = mlContext.Ranking.Trainers.LightGbm(labelColumnName: "Label", featureColumnName: "Features", rowGroupColumnName: "GroupId");
IEstimator trainerPipeline = dataPipeline.Append(trainer);
```
```C#
// Domain Model
public class Candidate
{
[LoadColumn(0)]
public string HIGHESTEDUCATION { get; set; }

[ColumnName("Label"),LoadColumn(1)]
public Single RELEVANCESCORE { get; set; }

[LoadColumn(2)]
public string SKILLSET { get; set; }

[LoadColumn(3)]
public string SOURCE { get; set; }

[LoadColumn(4)]
public string TOWNCITY { get; set; }

[ ColumnName("GroupId"), LoadColumn(5)]
public string VACANCYID { get; set; }

[LoadColumn(6)]
public string YEARSOFEXPERIENCE { get; set; }

}
```
Please paste or attach the code or logs or traces that would be helpful to diagnose the issue you are reporting.