WalletSynchronizer had a race condition caused by calling the callback before the Qt event loop (or other initialisation) finished. Ergo we split initialisation and the running of the thread, then use Qt SIGNALs to yield back into the Qt event loop. This ensures that the callback for the servers_list_changed is not called until the main Qt event loop is actually running.

lib/gui_qt.py

@@ -1430,6 +1430,9 @@ class ElectrumGui:
         if app is None:
             self.app = QApplication(sys.argv)
 
+    def server_list_changed(self):
+        pass
+
     def waiting_dialog(self):
 
         s = Timer()