Fix a crash when root view controller have to be defined by developer using setRootViewController

[rs]

This bug was introduced by 0e3dd3e680d7d5126efdf07dd8c527dfe5546a7f

To reproduce, just use the library in a way it can't determine the root view controller by itself and set it using advised [[SHK currentHelper] setRootViewController:...]; then use a service which require more than one modal view (i.e.: unlogged twitter in xauth mode).