Fix segfault crash when shutdown the GUI in disablewallet mode

Author: jonasschnelli

src/qt/bitcoingui.cpp

@@ -518,7 +518,10 @@ void BitcoinGUI::setClientModel(ClientModel *_clientModel)
         // Propagate cleared model to child objects
         rpcConsole->setClientModel(nullptr);
 #ifdef ENABLE_WALLET
-        walletFrame->setClientModel(nullptr);
+        if (walletFrame)
+        {
+            walletFrame->setClientModel(nullptr);
+        }
 #endif // ENABLE_WALLET
         unitDisplayControl->setOptionsModel(nullptr);
     }