The scalastyle Eclipse plugin silently dies with a NullPointerException ...

...for linked source files.

The Eclipse plugin uses getFileForLocation() to locate source files to
be scalastyled. Unfortunately, that API contains the following:
 * Warning: This method ignores linked resources and their children.  Since
 * linked resources may overlap other resources, a unique mapping from a
 * file system location to a single resource is not guaranteed.  To find all
 * resources for a given location, including linked resources, use the method
 * <code>findFilesForLocation</code>.
which causes:
java.lang.NullPointerException
	at org.scalastyle.scalastyleplugin.builder.ScalastyleBuilder.org$scalastyle$scalastyleplugin$builder$ScalastyleBuilder$$fileSpec(ScalastyleBuilder.scala:159)
	at org.scalastyle.scalastyleplugin.builder.ScalastyleBuilder$$anonfun$handleBuildSelection$1$$anonfun$2.apply(ScalastyleBuilder.scala:149)
	at org.scalastyle.scalastyleplugin.builder.ScalastyleBuilder$$anonfun$handleBuildSelection$1$$anonfun$2.apply(ScalastyleBuilder.scala:149)
 ...
for source files that are linked in to the project workspace.

If getFileForLocation() does return null, use
findFilesForLocationURI() and throw an exception if there is more than
one.