Added missing activeStack changed signal

This fixes some issues with the recommended view when the first machine is added.

CURA-4403

cura/Settings/MachineManager.py

@@ -304,6 +304,7 @@ class MachineManager(QObject):
                 quality.nameChanged.connect(self._onQualityNameChanged)
 
                 self._active_container_stack = self._global_container_stack
+                self.activeStackChanged.emit()
 
         self._error_check_timer.start()