Fix crash while re-entering the firewall KCM

kcm/core/firewallclient.cpp

@@ -32,6 +32,12 @@ FirewallClient::FirewallClient(QObject *parent)
 {
 }
 
+FirewallClient::~FirewallClient() noexcept
+{
+    m_currentBackend->deleteLater();
+    m_currentBackend = nullptr;
+}
+
 QStringList FirewallClient::knownProtocols()
 {
     if (!m_currentBackend) {

kcm/core/firewallclient.h

@@ -53,6 +53,8 @@ public:
     enum DefaultDataBehavior { DontReadDefaults, ReadDefaults };
     enum ProfilesBehavior { DontListenProfiles, ListenProfiles };
     explicit FirewallClient(QObject *parent = nullptr);
+    ~FirewallClient();
+
     void setBackend(const QStringList &backendList);
 
     Q_INVOKABLE static QStringList knownProtocols();