When an error check finishes, only trigger auto-slicing if needed

CURA-4092

Many things can trigger an error check, but Cura should only auto-slice
if the error check was scheduled due to a value change.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -453,6 +453,7 @@ class CuraEngineBackend(QObject, Backend):
 
     def _onStackErrorCheckFinished(self):
         self._is_error_check_scheduled = False
+        if self._need_slicing:
             self.needsSlicing()
             self._onChanged()