rewrote the Camera.LookAt get accessor to ensure that

when the Camera as a body resource does not define a value for a LookAt property,
then the value returned is null

src/Camera.ts

@@ -69,17 +69,20 @@ export class Camera extends AnnotationBody {
  * with an id matching the id of an Annotation instance.
  **/
  getLookAt() : object | PointSelector | null {
- let rawObj = this.getPropertyAsObject("lookAt" )
- if ( ! rawObj) return null;
+ let rawObj = this.getPropertyAsObject("lookAt" ) ?? null;
+ if ( rawObj == null ) return null;
 
- let rawType = (rawObj["type"] || rawObj["@type"])
- if (rawType == "Annotation"){
+ let rawType = (rawObj["type"] || rawObj["@type"]) ?? null;
+ if (rawType == null ) return null;
+
+ if (rawType == "Annotation")
  return rawObj;
- }
- if (rawType == "PointSelector"){
+ else if (rawType == "PointSelector")
  return new PointSelector(rawObj);
+ else{
+ console.error('unidentified value of lookAt ${rawType}');
+ return null;
  }
- throw new Error('unidentified value of lookAt ${rawType}');
  } 
  get LookAt() : object | null {return this.getLookAt();}