[Loadflow validation] NPE

[sylvlecl]

• Do you want to request a feature or report a bug?

Bug.

• What is the current behavior?

The validation of transformers can end up with a NPE:

13:55:15.267 [main] DEBUG com.powsybl.python.PyPowsyblApiLib - null
java.lang.NullPointerException: null
	at com.powsybl.dataframe.loadflow.validation.Validations.lambda$twtsValidationMapper$1(Validations.java:165)
	at com.powsybl.dataframe.StringSeriesMapper.createSeries(StringSeriesMapper.java:51)
	at com.powsybl.dataframe.AbstractDataframeMapper.lambda$createDataframe$1(AbstractDataframeMapper.java:49)
	at com.google.common.collect.CollectSpliterators$1WithCharacteristics.lambda$forEachRemaining$1(CollectSpliterators.java:71)
	at java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Streams.java:104)
	at com.google.common.collect.CollectSpliterators$1WithCharacteristics.forEachRemaining(CollectSpliterators.java:71)
	at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:658)
	at com.powsybl.dataframe.AbstractDataframeMapper.createDataframe(AbstractDataframeMapper.java:49)
	at com.powsybl.python.Dataframes.createCDataframe(Dataframes.java:70)
	at com.powsybl.python.PyPowsyblLoadFlowValidationApiLib.createCDataFrame(PyPowsyblLoadFlowValidationApiLib.java:103)
	at com.powsybl.python.PyPowsyblLoadFlowValidationApiLib.createLoadFlowValidationSeriesArray(PyPowsyblLoadFlowValidationApiLib.java:47)
	at com.powsybl.python.PyPowsyblLoadFlowValidationApiLib.lambda$createLoadFlowValidationSeriesArray$0(PyPowsyblLoadFlowValidationApiLib.java:41)
	at com.powsybl.python.Util.doCatch(Util.java:76)
	at com.powsybl.python.PyPowsyblLoadFlowValidationApiLib.createLoadFlowValidationSeriesArray(PyPowsyblLoadFlowValidationApiLib.java:39)