Avoids crash when changing device's orientation and pressing back button on the Wi-Fi sharing screen.

E/AndroidRuntime(  390): Caused by: java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState

E/AndroidRuntime(  390): Caused by: java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
E/AndroidRuntime(  390): 	at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1343)
E/AndroidRuntime(  390): 	at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1354)
E/AndroidRuntime(  390): 	at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:595)
E/AndroidRuntime(  390): 	at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:574)
E/AndroidRuntime(  390): 	at com.frostwire.android.gui.activities.MainActivity.switchContent(MainActivity.java:604)
E/AndroidRuntime(  390): 	at com.frostwire.android.gui.activities.MainActivity.switchContent(MainActivity.java:634)
E/AndroidRuntime(  390): 	at com.frostwire.android.gui.activities.internal.MainController.switchFragment(MainController.java:75)
E/AndroidRuntime(  390): 	at com.frostwire.android.gui.activities.MainActivity.refreshPeersFragment(MainActivity.java:599)
E/AndroidRuntime(  390): 	at com.frostwire.android.gui.activities.MainActivity.onResume(MainActivity.java:311)