Thank you very much for the guidance you provided. I will definitely implement the point you mentioned in my module.
The plans I have in mind are to implement modules with names such as VolumeSelector, ModelSelector, BeamSelector, etc., separately, in order to expand my experience in module development.
However, if you pay attention to the following figure, you will notice that the parent class used is of type qMRMLWidget. Interestingly, the qMRMLSegmentSelectorWidget widget does not have the nodeType property.
But as mentioned you, if a widget of type qMRMLNodeComboBox is selected, it does have the nodeType property, and I think I should set it to vtkMRMLScalarVolumeNode or vtkMRMLSegmentationNode in python code.
Best regards.
Shahrokh