Why does Mr. Frank say, "I beg you, don't let [Miep] see a thing like this."?
He is embarrassed by what she will think if she sees them all fighting.
He is angry that she came in the middle of the night while they are all in their pajamas.
He is scared that she has brought the SS with her.
He is worried that she will discover the plans they have made to blow up the building.