Fix crash on "Set up Wi-Fi NFC tag" dialog

When user launches "Set up Wi-Fi NFC tag" dialog
and then tries to rotate screen, device will
crash.
Because isShowing() of dialog won't return true
anymore when fragment calls onSaveIntance(),
we can't save status of dialog successfully.
And then, when fragment called onCreateDialog()
again, it can't get any dialog object.
So, we only check dialog whether is null or not.
If dialog is null, it means that there is no dialog
was shown before user rotates the screen.

Fixes: 112741721
Test: NFC tag wifi test, robo test

Change-Id: Idb448ea32c4215d8380c69bfd896cc91d8c1f8d1