Index out of range (-1) when I click on any item in the TListBox

The TListBox called lboMtrlList is populated with records from the database. The data displays properly when I run the application. When I click on any item in the list, shows the error:

Index out of range (-1)

despite the list not being empty.

Here's the code for populating the lboMtrlList:

procedure TfrmMakeQuote.FormCreate(Sender: TObject);
 begin
  con := TFDConnection.Create(nil);
  query := TFDQuery.Create(con);
  con.LoginPrompt := false;
  con.Open('DriverID=SQLite;Database=C:\Users\katiee\Documents\Embarcadero\' +
   'Studio\Projects\ProgramDatabase;');
  query.Connection := con;
  performQuery;

  query.SQL.Text :=
  'SELECT [Material Description] FROM MtrlDatabase ORDER BY MtrlID';
 try
  query.Open;
  lboMtrlList.Items.Clear;
  while not query.EOF do
   begin
    lboMtrlList.Items.Add(query.Fields[0].AsString);
    query.Next;
   end;
  finally
   query.Close;
 end;
 //ledtDesc.Height :=  81;
 //ledtNotes.Height :=  51;
 end;

I want to be able to double click on an item in the lboMtrlList and move it to another TListBox called lboSelectedMtrl. Here's the code:

procedure TfrmMakeQuote.lboMtrlListDblClick(Sender: TObject);
 begin
  lboMtrlList.Items.Add(lboSelectedMtrl.Items.Strings[lboSelectedMtrl.ItemIndex]);
 end;