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)


return this.contentType === "text/html";
  }
  [$onChild](child) {
    if (
      this.contentType === "text/html" &&
      child[$namespaceId] === NamespaceIds.xhtml.id
    ) {
      this[$content] = child;
      return true;
    }
    if (this.contentType === "text/xml") {
      this[$content] = child;
      return true;
    }
    return false;
  }
  [$toHTML](availableSpace) {
    if (this.contentType !== "text/html" || !this[$content]) {
      
      return HTMLResult.EMPTY;
    }
    return this[$content][$toHTML](availableSpace);
  }
}
class ExObject extends XFAObject {