Change manage affiliations to check if window is nil

[mimi89999]

This can happen if the window was closed before the error is received

To reproduce the issue open manage affiliations on a room where you don't have permissions to do that and immediately after press escape.

I submit my source code and allow Tigase to release the code as a part of Tigase's own software package under any license, either an open source or closed source.

[hantu85]

I think replacing this whole if with guard-else would be a better fit but this fix is OK.