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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Steps to Reproduce the Issue

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Expected Outcomes

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