fix: fix ddm crash on treeland crash

The problem is caused by reckless cleanup process. Check those pointers
carefully to make reconnection works.

Author: calsys456

src/daemon/TreelandConnector.cpp

@@ -220,10 +220,14 @@ void TreelandConnector::connect(QString socketPath) {
 
 void TreelandConnector::disconnect() {
     if (m_display) {
-        m_notifier->setEnabled(false);
+        if (m_notifier)
+            m_notifier->setEnabled(false);
         wl_display_disconnect(m_display);
         delete m_notifier;
+        m_display = nullptr;
     }
+    if (m_ddm)
+        m_ddm = nullptr;
 }
 
 // Request wrapper