gui: guard accessing a nullptr 'clientModel'

During shutdown, already queue events dispatched from the backend such
'numConnectionsChanged' and 'networkActiveChanged' could try to access
the clientModel object, which might not exist because we manually delete
it inside 'BitcoinApplication::requestShutdown()'.

src/qt/clientmodel.h

@@ -58,6 +58,8 @@ public:
     explicit ClientModel(interfaces::Node& node, OptionsModel *optionsModel, QObject *parent = nullptr);
     ~ClientModel();
 
+    void stop();
+
     interfaces::Node& node() const { return m_node; }
     OptionsModel *getOptionsModel();
     PeerTableModel *getPeerTableModel();