Why was Yeshaya at the door? (Eruvin 26a)

The gemara concludes that you should not daven at someone's door for them b/c it could entice the Satan (malach hamaves) to come even though Yeshaya davened at Chizkiya's backdoor. The Maharsha asks two obvious questions on this gemara.
1. If you shouldn't do it why was it good enough for Yeshaya?
2. Why was he at the backdoor?

I thought that one question could answer the other. You shouldn't do it at the front door but the backdoor is OK. Not a great answer but it would have been enough for me.

Thankfully though the Maharsha had a much better answer. We can't daven at someone's door because maybe the satan wasn't planning on coming and we will change his mind. Yeshaya, on the other hand, KNEW that the satan was planning on killing Chizkiya so it was OK for him. He was at the backdoor because he also knew that was how the Satan was planning on entering. Helps to be a navi.