TWebClientDataset.FieldByName and FFieldMapper is null

I have a TWebClientDataSet which retrieves data, the data is displayed in a grid, so I know, that it is there.

  Webmemo1.Lines.Add(WebClientConnection2.URI);
  WebClientConnection2.Active:=true;
  WebMemo1.Lines.add(WebClientDataSet2.FieldByName('NAEHRWERTCD').AsString+' '+WebClientDataSet2.FieldByName('WERT')

However the last line results in:

ERROR
TypeError: this.FFieldMapper is null | this.GetFieldData$1@https://www.nutritional-software.at/WebCore/P03/Project1.js:99045:7 this.GetFieldData@https://www.nutritional-software.at/WebCore/P03/Project1.js:90189:21 this.GetData@https://www.nutritional-software.at/WebCore/P03/Project1.js:86636:32 this.GetAsString@https://www.nutritional-software.at/WebCore/P03/Project1.js:86758:16 this.WebDBGrid1ClickCell@https://www.nutritional-software.at/WebCore/P03/Project1.js:135635:82 cb@https://www.nutritional-software.at/WebCore/P03/Project1.js:242:19 this.HandleClick@https://www.nutritional-software.at/WebCore/P03/Project1.js:123394:14 cb@https://www.nutritional-software.at/WebCore/P03/Project1.js:254:28
at https://www.nutritional-software.at/WebCore/P03/Project1.js [99045:7]

The fields are persistent.

There is another CDS on the form, where the same approach works as expected.

What am I missing?

The whole event looks like this:

procedure TForm1.WebDBGrid1ClickCell(Sender: TObject; ACol, ARow: Integer);
var
  s1, s2: string;
  sSearch: string;
begin
  WebMemo1.Lines.Clear;
  s1 := WebClientDataSet1.FieldByName('Lebensmittelcd').AsString;
  s2 := WebClientDataSet1.FieldByName('herkunftcd').AsString;
  WebMemo1.Lines.Add(s1 + '-' + s2);
  sSearch := '?filter=LEBENSMITTELCD,eq,' + s1 + '&HERKUNFTCD,eq,' + s2;;
  WebClientConnection2.URI := 'https://www.nutritional-software.at/api/api.php/records/TBLNAEHRWERT' + sSearch;
  Webmemo1.Lines.Add(WebClientConnection2.URI);
  WebClientConnection2.Active:=true;
  WebMemo1.Lines.add(WebClientDataSet2.FieldByName('NAEHRWERTCD').AsString+' '+WebClientDataSet2.FieldByName('WERT').AsString);  < ----------------- fails
end;

Same pitfall as here: