BufferBuilder.MakeDense: IndexOutOfRangeException

**System Information:**
- OS & Version: Windows 11
- ML.NET Version: 1.6.0 (custom build with autoML search space edited)
- .NET Version: .NET 5.0

**Describe the bug**
I am running AutoML. The below is from logs, probably about 1 out of 50 runs on same dataset.

**To Reproduce**
Unknown. It might be related to specific model parameters, but they were not logged (the search space is customized).
Unfortunately, not even model is saved, since I am only exploring the weights. Model was trained succesfully.

**Screenshots, Code, Sample Projects**
Although, global feature index works almost always, I have this exception in logs.

```
System.IndexOutOfRangeException: Index was outside the bounds of the array.
at Microsoft.ML.Data.BufferBuilder`1.MakeDense() in Q:\git-kwork-microsoft-ml\Microsoft.ML\src\Microsoft.ML.Data\Data\BufferBuilder.cs:line 322
at Microsoft.ML.Data.BufferBuilder`1.GetResult(VBuffer`1& buffer) in Q:\git-kwork-microsoft-ml\Microsoft.ML\src\Microsoft.ML.Data\Data\BufferBuilder.cs:line 462
at Microsoft.ML.Trainers.FastTree.TreeEnsembleModelParameters.GetFeatureWeights(VBuffer`1& weights) in Q:\git-kwork-microsoft-ml\Microsoft.ML\src\Microsoft.ML.FastTree\FastTree.cs:line 3242
at Kwork.AI.AutoML.Experiment.SaveGlobalFeatureIndexFastTree(MLContext mlContext, ITransformer model, String filename, IDataView testDataView, BinaryClassificationTrainer trainer) in Q:\git-kwork-microsoft-ml\Microsoft.ML\src-AutoML-Runner\Kwork.AI.AutoML.Runner\Experiment.cs:line 668
```

Link to BufferBuilder.MakeDense: https://github.com/dotnet/machinelearning/blob/52ddbcd0a57b440e60e43e6cb39b49fe55bdfe55/src/Microsoft.ML.Data/Data/BufferBuilder.cs#L316

**Additional context**
It is not blocking my work, only for FYI