fixed https://github.com/JabRef/jabref/issues/6472
fixed https://github.com/JabRef/jabref/issues/6472

fixed the problem that multiple background task popups stacked over each other.

Added a globle PopOver variable to hide existed PopOver(if any).

Siedlerchr/jabref,sauliusg/jabref,Siedlerchr/jabref,Siedlerchr/jabref,JabRef/jabref,sauliusg/jabref,Siedlerchr/jabref,JabRef/jabref,JabRef/jabref,sauliusg/jabref,JabRef/jabref,sauliusg/jabref