Invoking Overlay.isReference(Schema,Key) gives a NullPointer if key does not exist

[MikeSchumacherCapgemini]

When trying to find out whether a property is a reference or not, or getting the reference, if the key does not exist, a NullPointer is thrown. Perhaps it would be better to return false or null.

Stacktrace:

java.lang.NullPointerException
	at com.reprezen.jsonoverlay.PropertiesOverlay.getReference(PropertiesOverlay.java:209)
	at com.reprezen.jsonoverlay.Overlay.getPropertyReference(Overlay.java:309)
	at com.reprezen.jsonoverlay.Overlay.getReference(Overlay.java:283)
	at com.reprezen.jsonoverlay.Overlay.isReference(Overlay.java:266)
	at com.reprezen.jsonoverlay.Overlay.isReference(Overlay.java:270)

Example:

if(Overlay.isReference(someSchema,"items")){
     some code
}

returns null when someSchema is like

{"type":"array","items":{"$ref":"#/components/schemas/MoreData"},"description":"a single ref to MoreData (many-to-one)"}

@maybeec