DELPHI - How show modal form

TFMissing is class with form.

var
  FMissing : TFMissing;
begin
  FMissing := TFMissing.Create( self );
  
  { -- optional: here you can send some initial data to the form  
  FMissing.Prepare( DataColumn );
  }
  
  FMissing.ShowModal;

  { -- optional: here you have possibility read output from (closed) form   
  Grid.Cells[ _iCol, _iRow ] := pDataColumn.pMissings.GetAsString; 
  }

  FMissing.Free;
end;

DELPHI - How properly close non-modal window

Override event FormClose:

procedure TFGauge.FormClose( Sender: TObject; var Action: TCloseAction );
begin
  Action := caFree;
  self   := nil;
end;