I agree that the moment is incorrect. I believe we can assume that [imath]N_1=0[/imath]. Indeed, in the case when [imath]P[/imath] is gradually increased and eventually topples the block we'll pass through the moment when the right bottom edge is lifted of the surface by infinitely small amount so that the geometry is essentially the same, but there is no longer any weight applied to the right bottom edge.
Does this make sense?