Attach (recommended) or Link to PDF file here:
issue13500 (in test corpus)

This regressed when landing #13566. 

What is the expected behavior? (add screenshot)
<img width="664" alt="image" src="https://user-images.githubusercontent.com/942640/122467522-7bc8aa80-cf6f-11eb-9d4d-311eb7c64fb2.png">


What went wrong? (add screenshot)
![image](https://user-images.githubusercontent.com/942640/122467565-85eaa900-cf6f-11eb-9c10-8a8686ed440c.png)


this.occur = null;
    this.overflow = null;
    this.pageSet = null;
    this.para = null;
    this.traversal = null;
    this.validate = null;
    this.variables = null;
    this.area = new XFAObjectArray();
    this.breakAfter = new XFAObjectArray();
    this.breakBefore = new XFAObjectArray();
    this.connect = new XFAObjectArray();
    this.draw = new XFAObjectArray();
    this.event = new XFAObjectArray();
    this.exObject = new XFAObjectArray();
    this.exclGroup = new XFAObjectArray();
    this.field = new XFAObjectArray();
    this.proto = new XFAObjectArray();
    this.setProperty = new XFAObjectArray();
    this.subform = new XFAObjectArray();
    this.subformSet = new XFAObjectArray();
  }
  *[$getContainedChildren]() {
    
    
    yield* getContainedChildren(this);
  }
  [$flushHTML]() {
    return flushHTML(this);
  }
  [$addHTML](html, bbox) {