I solved the problem by following the advice of placing form2 PopupParent: = Form1.
Other way:
Set for TForm2 in designer: PopupMode:=pmAuto
I think this way is better, because it works for all instances of TForm2 and does not require an explicit code for each call.
In general, PopupParent should not be set in designer because:
1. Different forms (classes and instanses) may call this form.
2. Global variable may be not used at all.