Parsing an empty file cause NullPointerException

[jmini]

When you parse an empty file you get this:

Exception in thread "main" java.lang.NullPointerException
	at java.util.ArrayDeque.addLast(ArrayDeque.java:249)
	at java.util.ArrayDeque.add(ArrayDeque.java:423)
	at com.reprezen.jsonoverlay.Resolver.treeWalk(Resolver.java:97)
	at com.reprezen.jsonoverlay.Resolver.findReferenceNodes(Resolver.java:92)
	at com.reprezen.jsonoverlay.Resolver.preresolveBase(Resolver.java:67)
	at com.reprezen.jsonoverlay.Resolver.preresolve(Resolver.java:58)
	at com.reprezen.jsonoverlay.Resolver.preresolve(Resolver.java:50)
	at com.reprezen.kaizen.oasparser.OpenApiParser.parse(OpenApiParser.java:96)
	at com.reprezen.kaizen.oasparser.OpenApiParser.parse(OpenApiParser.java:90)
	at com.reprezen.kaizen.oasparser.OpenApiParser.parse(OpenApiParser.java:67)
	at com.reprezen.kaizen.oasparser.OpenApi3Parser.parse(OpenApi3Parser.java:43)

A more user-friendly Error would be better.