Fix softlock caused by backing out of the "Recovery Phrase" view while restoring a Xelis wallet on Linux

[sneurlax]

fix: check isOnline before offlineMode in Xelis exit()

Creating a Xelis wallet on Linux but backing out at the "Recovery Phrase" page throws

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception:
AnyhowException(Wallet is not in online mode)
#0      SimpleDecoder.decode
(package:flutter_rust_bridge/src/codec/base.dart:32)
#1      DcoCodec.decodeObject
(package:flutter_rust_bridge/src/codec/dco.dart:25)
<asynchronous suspension>
#2      LibXelisWallet.exit.<anonymous closure> (package:stackwallet/
ets/wallet/intermediate/lib_xelis_wallet.dart:173)
<asynchronous suspension>
#3      Mutex.protect (package:mutex/src/mutex.dart:79)
<asynchronous suspension>
#4      LibXelisWallet.exit (package:stackwallet/wallets/wallet/
ate/lib_xelis_wallet.dart:165)
<asynchronous suspension>
#5      _NewWalletRecoveryPhraseViewState.delete
(package:stackwallet/pages/add_wallet_views/
ew/new_wallet_recovery_phrase_view.dart:85)
<asynchronous suspension>
#6      _NewWalletRecoveryPhraseViewState.build.<anonymous closure>
(package:stackwallet/pages/add_wallet_views/
ew/new_wallet_recovery_phrase_view.dart:116)
<asynchronous suspension>

didChangeAppLifecycleState: inactive

and softlocks you onto that page. Neither the back button nor the "Exit to My Stack" button work to let you exit the page

This fixes that