Fixes for 2 issues

Fixes for 2 small issues:
1. By disallowing focusing on static labels, the next best GUI component that will get focus is the current password field. In case a faulty password is entered, the password dialog reappears only the current password field isn't focused. By making these changes, the password field *will* be focused after reappearing, which is less annoying for heavy keyboard users.
2. When a user does not have any accounts configured and he then clicks the 'Add chat room' menu item, an NPE will occur, since the code assumes that an element is selected in the account combobox. However, since there are no accounts available, the combobox selection will return null, resulting in a NPE. Now checks instance instead of assuming existence.