sometimes bug with guessSource

[AlexanderPico]

NPE while checking first 10 IDs to guess source.

Steps to (sometimes) reproduce:

1. open galfiltered
2. map Ensembl to Entrez
3. attempt to map from Entrez
   ==> NPE

Perhaps add a check for NULL here and skip?

https://github.com/cytoscape/idmapper/blob/master/src/main/java/org/cytoscape/idmapper/MappingSource.java#L151

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at org.cytoscape.idmapper.MappingSource.guessSource(MappingSource.java:151)
at org.cytoscape.idmapper.task.ColumnMappingTask.guessSource(ColumnMappingTask.java:134)
at org.cytoscape.idmapper.task.ColumnMappingTask.resetSpecies(ColumnMappingTask.java:111)
at org.cytoscape.idmapper.task.ColumnMappingTask.speciesSelectionChanged(ColumnMappingTask.java:83)
at org.cytoscape.idmapper.task.ColumnMappingTask.selectionChanged(ColumnMappingTask.java:343)