From the grammar tips section:
Pronominal adverbs with 'er', 'hier', 'daar' or 'waar' are usually spelled as one word. However, in many cases they can be split up, for example by another adverb or by an indefinite object (see the Word Order 1 skill). Occasionally this is even mandatory (e.g. with an indefinite subject). Some examples:
Wat zit er nu in? - What is in there now? Er zit niets in. - There is nothing in it. Ik stop er iets in. - I put something in it.