\itemsep to change the spacing doesn't work in presentation class file

Temporary solution (credits: Christoph)

\usepackage{xpatch}

\xpatchcmd{\itemize}
{\def\makelabel}
{\ifnum@itemdepth=1\relax
\setlength\itemsep{2em}
\else
\ifnum@itemdepth=2\relax
\setlength\itemsep{2ex}
\fi\fi\def\makelabel
}
{}
{}