Program: 142283
Project: 142283
Buggy source:
public AbstractTreeElement generateRoot(ExternalDataInstance instance){
  CommCareApplication app=CommCareApplication._();
  String ref=instance.getReference();
  if (ref.indexOf(LedgerInstanceTreeElement.MODEL_NAME) != -1) {
    if (stockbase == null) {
      SqlStorage<Ledger> storage=app.getUserStorage(Ledger.STORAGE_KEY,Ledger.class);
      stockbase=new LedgerInstanceTreeElement(instance.getBase(),storage);
    }
 else {
      stockbase.rebase(instance.getBase());
    }
    return stockbase;
  }
 else   if (ref.indexOf(CaseInstanceTreeElement.MODEL_NAME) != -1) {
    if (casebase == null) {
      SqlStorage<ACase> storage=app.getUserStorage(ACase.STORAGE_KEY,ACase.class);
      casebase=new AndroidCaseInstanceTreeElement(instance.getBase(),storage,false);
    }
 else {
      casebase.rebase(instance.getBase());
    }
    return casebase;
  }
 else   if (instance.getReference().indexOf("String_Node_Str") != -1) {
    String userId="String_Node_Str";
    User u=CommCareApplication._().getSession().getLoggedInUser();
    if (u != null) {
      userId=u.getUniqueId();
    }
    String refId=ref.substring(ref.lastIndexOf('/') + 1,ref.length());
    FormInstance fixture=CommCareUtil.loadFixture(refId,userId);
    if (fixture == null) {
      throw new RuntimeException("String_Node_Str" + ref);
    }
    TreeElement root=fixture.getRoot();
    root.setParent(instance.getBase());
    return root;
  }
  if (instance.getReference().indexOf("String_Node_Str") != -1) {
    User u=app.getSession().getLoggedInUser();
    TreeElement root=session.getSessionInstance(app.getPhoneId(),app.getCurrentVersionString(),u.getUsername(),u.getUniqueId(),u.getProperties()).getRoot();
    root.setParent(instance.getBase());
    return root;
  }
  return null;
}

Fixed version (ground truth):
public AbstractTreeElement generateRoot(ExternalDataInstance instance){
  CommCareApplication app=CommCareApplication._();
  String ref=instance.getReference();
  if (ref.indexOf(LedgerInstanceTreeElement.MODEL_NAME) != -1) {
    if (stockbase == null) {
      SqlStorage<Ledger> storage=app.getUserStorage(Ledger.STORAGE_KEY,Ledger.class);
      stockbase=new LedgerInstanceTreeElement(instance.getBase(),storage);
    }
 else {
      stockbase.rebase(instance.getBase());
    }
    return stockbase;
  }
 else   if (ref.indexOf(CaseInstanceTreeElement.MODEL_NAME) != -1) {
    if (casebase == null) {
      SqlStorage<ACase> storage=app.getUserStorage(ACase.STORAGE_KEY,ACase.class);
      casebase=new AndroidCaseInstanceTreeElement(instance.getBase(),storage,false);
    }
 else {
      casebase.rebase(instance.getBase());
    }
    return casebase;
  }
 else   if (instance.getReference().indexOf("String_Node_Str") != -1) {
    String userId="String_Node_Str";
    User u=CommCareApplication._().getSession().getLoggedInUser();
    if (u != null) {
      userId=u.getUniqueId();
    }
    String refId=ref.substring(ref.lastIndexOf('/') + 1,ref.length());
    try {
      FormInstance fixture=CommCareUtil.loadFixture(refId,userId);
      if (fixture == null) {
        throw new RuntimeException("String_Node_Str" + ref);
      }
      TreeElement root=fixture.getRoot();
      root.setParent(instance.getBase());
      return root;
    }
 catch (    IllegalStateException ise) {
      throw new RuntimeException("String_Node_Str" + ref);
    }
  }
  if (instance.getReference().indexOf("String_Node_Str") != -1) {
    User u=app.getSession().getLoggedInUser();
    TreeElement root=session.getSessionInstance(app.getPhoneId(),app.getCurrentVersionString(),u.getUsername(),u.getUniqueId(),u.getProperties()).getRoot();
    root.setParent(instance.getBase());
    return root;
  }
  return null;
}