Return empty LookupResult if lookup range beyond body

lib/models/documents/nodes/container.dart

@@ -59,11 +59,8 @@ abstract class Container<T extends Node> extends Node {
   }
 
   ChildQuery queryChild(int offset, bool inclusive) {
-    if (offset < 0) {
-      throw ('Offset cannot be negative');
-    }
-    if (offset > length) {
-      throw ('offset $offset > $length');
+    if (offset < 0 || offset > length) {
+      return ChildQuery(null, 0);
     }
 
     for (Node node in children) {