LyX – undefined control sequence

I just started to explore the new LyX and soon enough I have encountered mysterious error, when creating presentation in using Beamer. I have started with the English Beamer template and soon as I got comfortable with LyX (being devout user of Emacs) I have deleted rest of the template and continued on my own.
What I haven’t noticed, was a deleted EndFrame, which was causing „Undefined control sequence“ on all section titles.