SparkRowConverter for ImagingStudy will throw NPE on DSTU3

[anyili]

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Description of Issue

 I am initializing the spark row converter 
 implicit val converter: SparkRowConverter = {
  SparkRowConverter.forResource(FhirContexts.forStu3(), "ImagingStudy")
}
Then it threw 

Caused by: java.lang.NullPointerException
at com.cerner.bunsen.spark.converters.DefinitionToSparkVisitor.lambda$visitComposite$0(DefinitionToSparkVisitor.java:442)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:546)
at java.util.stream.AbstractPipeline.evaluateToArrayNode(AbstractPipeline.java:260)
at java.util.stream.ReferencePipeline.toArray(ReferencePipeline.java:505)
at com.cerner.bunsen.spark.converters.DefinitionToSparkVisitor.visitComposite(DefinitionToSparkVisitor.java:445)
at com.cerner.bunsen.spark.converters.DefinitionToSparkVisitor.visitComposite(DefinitionToSparkVisitor.java:47)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.elementToFields(Stu3StructureDefinitions.java:353)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.transformChildren(Stu3StructureDefinitions.java:424)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.elementToFields(Stu3StructureDefinitions.java:350)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.transformChildren(Stu3StructureDefinitions.java:424)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.transformRoot(Stu3StructureDefinitions.java:618)
at com.cerner.bunsen.definitions.stu3.Stu3StructureDefinitions.transform(Stu3StructureDefinitions.java:538)
at com.cerner.bunsen.spark.SparkRowConverter.forResource(SparkRowConverter.java:97)
at com.cerner.bunsen.spark.SparkRowConverter.forResource(SparkRowConverter.java:54)

System Configuration

in build.gradle

implementation 'com.cerner.bunsen:bunsen-core-stu3:0.5.11'
implementation 'com.cerner.bunsen:bunsen-core:0.5.11'
implementation 'com.cerner.bunsen:bunsen-spark-stu3:0.5.11'
implementation 'com.cerner.bunsen:bunsen-spark:0.5.11'

Project Version

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1. Step 1
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Expected Outcomes

• A description of how the system is expected to behave after the issue is resolved.